I've got it early and have been playing around on the game all day, so here's my first impressions so far. I've been using Wesker/Skrull, Doctor Doom and Dormammu/Sentinel.
Captain America - very balanced in all ways, he has very easy to connect combos that can be extended with Shield Slash making for some nice damage. His supers are also very easy to hit confirm into aswell.
Deadpool - you win just for picking this guy, he's just hillarious. Haven't used too much of him but the gun super seems pretty easy to hit with, since Katanarama makes a wall bounce straight into it. After an air combo you can simply use the aerial gun super to shoot them while they're on the ground.
Doctor Doom - wow, this guy has been on pretty much every team so far, he's amazing, I might even go to say he's better than Mvc2. Plasma Beam has huge priority and seems to beat all other beams/guns/fireballs out there, causes tons of hit stun, chip damage and pushes the opponent way back. Molecular Shield is just as good as it was in 2, if not better. It's still a crazy good assist and can put incredible pressure on opponents in the corner. I think Doom is going to be pretty high up the tiers in this game. The only downside to doom is that his supers aren't very strong (exception of his level 3 of course), but Photon Array fills the screen and pushes the opponents right to the other side of the screen, exactly where Doom wants them to be. I can't get enough of this guy.
Dormammu - this guy just looks incredibly badass, and he has a seriously good keepaway game to boot. Down forward + H makes a flame carpet appear on the ground which is like one of Trish's traps, which is really good for zoning. From there he can power up his meteor attacks or just spam dark portals, and go straight into chaotic flame if he hits, doing nice damage. I'm sure everyone knows the Throwing Bomb, X factor cancelled into another Throwing bomb also does an unavoidable 50% chip damage. He's not bad at close combat either, unlike MODOK or things like Dhalsim from the SF games. Overall, a very durable character with awesome assists and damaging combos. He can be a nightmare to get inside, and just when you think you have, he can teleport away.
Hulk - he's a slow, heavy hitting truck of course. Not much else to say about him, except his aerial H is weird as it makes him bounce in the air.
Iron Man - I haven't really played with this guy, but he's solid in the hands of a CPU on Hard difficulty. The cannon hyper just seems to combo of of freaking everything, and Repulsor Blast is a very annoying attack, not to mention his UniBeams. A very good all around character.
Magneto - he's still solid, but nowhere as near as godlike as MvC2. In the hands of the computer he has an awesome rushdown game. I think his beam attack is the fastest in the game (but doesn't have the awesome qualities of Plasma Beam), so it can be spammed to high hell and generally be an asshole. I've mostly been playing with his level 3 - crazy move, it connects from absolutely anywhere, even if they're on the ground. Cool move.
MODOK - ugh, I hate this POS. He's so awkward to control and is completely useless once he's locked down. Maybe someone can make magic happen with him, but christ I hate him.
Phoenix - mixed feelings.. on one hand she's super strong with very damaging combos and amazing homing projectiles, but she really does die to just one combo. I did a simple L > M > H > launcher > L > M > super from Wesker, and she died. The only reason to use her imo is to abuse Dark Phoenix, otherwise go for another offensive character like Super Skrull. A CPU had DF on me once, and wow.. Doom was blocking a normal attack string and took 25% chip damage, absolutely nuts. There is projectiles and shit flying everywhere, the best you can do is try stalling her out and not dying to the ridiculous chip damage. Doom is pretty good at running away and defending himself with lasers / Photon Array.
Sentinel - the only guy in the game who is clearly not balanced. Sentinel has simple combos which do upwards of 60% damage, his crouching medium is the most broken thing ever, and he has bucketloads of health. The only nerf about him is rocket punch being a tad slower, but it can be cancelled into completely safe supers anyway. Unfortunately he's pretty much god tier again.
She-Hulk - not played much of her, but she has an instant dizzy grab which runs across about 3/4 screen, which sets up lots of damaging combos. Her level 3 is an unblockable grab, and she has a kind of dash like El Fuerte in SF4, where she can end it with different things, one being an OTG slide, one being a simple sucker punch to start a combo, and one which looks like guile's flash kick.
Spiderman - just a ninja as he was in MvC2, very fast with good combo possibilities with web swing, and Maximum Spider is very easy to connect.
Storm - I couldn't pull of her infinite combo (yay!). She's very fast and has whirlwinds make for a strong keepaway game, but she's no slouch in close either, ending air combos in lightning storm or hail supers. The hailstorm does loads of chip damage too.
Super Skrull - oh my god, this guy is incredible. A simple air combo does a lot of damage, and then he can use tenderizer (great move) to OTG them when they land, and then go straight into a super dealing massive damage. He's fast, he's crazy offensive and VERY difficult to guard against. His command grab literally reaches 3/4 of the screen away, and it's almost instant. If you land that (easy as hell), you can tenderizer again and go into another super! One of the best offensive characters in the game, without question. I think he'll be high tier.
Taskmaster - the best thing about tasky is that his normals has massive range, one of his normals makes his sword extend for about half the screen, and he has a very wide launcher too. Great Swing allows for huge combos in the air, and his super is great and hitting opponents on the floor. Overall very solid.
Thor - he's not bad at all. I got stuck on arcade against a Thor who just wouldn't let me win. Very damaging combos, and he has the second highest stamina in the game. His only drawback is being really slow, and it's kinda difficult for him to get his slow attacks in to start his big damage combos.
Wolverine - same old Wolverine, but with a dive kick. It's not that fast though.
X-23 - wow, she is fast. I think the fastest character in the game. She has some sick combos leading into easy OTG supers, and a very deadly hop grab move. Her level 3 is practically Shuma's Chaos Dimension, only you're invisible.
Akuma - strong! His hurricane kick is just AWESOME, it can lead into more combos leading into air combos, or go straight into supers. He does have low life, but that's okay since he deals out so much damage.
Wesker - this guy is just great in every way. His combos do lots of damage, and his super is just so damned easy to combo into (the one where he zooms around everywhere), and I'm pretty sure it can DHC from EVERYTHING. This guy is super stylish and powerful, and he has a very fast fast teleport which can make it difficult to tell which way to block. A character that's gone under the radar - he's really, really good.
Amaterasu - I haven't played with her that much, but her chain weapon hits the entire screen, turning her into freaking Dhalsim. The elements super seems really easy to combo into and is great for DHCs. She has an annoying hitbox which causes a lot of mvoes to simply go right over her head.
Arthur - kinda like MODOK.. I personally don't like him. I will say that his level 3 dragon super is one of the coolest in the game though.
Chris Redfield - the guy seems to have something for just about every single situation, form long range guns to shotguns, and keep away mines. He's also really good in close combat with his "karate attack" (well that's what he shouts when he does it) to make some fierce combos.
Chun-Li - another crazy fast fighter, her dash goes very far and feels almost insant. Comboing into her supers is a piece of cake and in the case of lightning legs, you can do an air combo after you've landed it. Solid character.
C. Viper - VERY offensive, much like Super Skrull in that her Seismo Hammer move is one of the best OTG moves in the game. Her hammers come out instantly and have practically no recovery - spam them to death and you can create some serious pressure. Supers are also dead easy to combo into from thunder knuckle.
Dante - yeah I know everyone is excited about him, but he has so many combos and specials that it'll take a while to learn. I'll come back to him...
Felicia - underrated! She's really fast and has great combos, with easy to land supers. She has a weird command dash that makes her pop out of the 2D plane she's fighting in and go around the back of her opponent, making her scarily agile. Worse part is the dash is completely invincible - think Blanka's hop from SF4. Check it out:
http://www.youtube.com/watch?v=qPoyAfYi1_E
Hsien-ko - she's a barrel of laughs to play, that chandelier swinging move is just so damned fun, and she can create shockwaves which reflect all enemy projectiles back in their faces. She's a bit slow though. My main issue with her is that I CAN NOT GET HER LAUNCHER TO WORK. Seriously, it will not even combo from L > launcher from standing in Sentinel's face. I don't get how you're supposed to use it, it's terrible.
Haggar - umm.. a bit TOO slow I think. His piledriver range is pretty bad and the lariat takes a long time to recover, making for easy punishments. He completely loses against any character with a decent projectile, i.e. Doom. He cannot get close to Doom at all.
Morrigan - eh.. average, she's nothing amazing. Her only quirk is her dash makes her fly up into the air, making for quick aerial attacks and a lot of pressure. The real reason to use Morrigan is her third assist - it gives you about 1/4 super meter every time she is called, which is simply stunning. She'll end up being a Captain Commando, in that she's not that great but her assist is amazing.
Spencer - oof, this guy packs a punch.. a very mobile heavy hitter with easy OTG into super combos means he can dish out the damage fast, and he's no glass cannon either. Difficult to take down and approach.
Ryu - your average Ryu. I really can't say much else..
Trish - an odd character. She can create craptons of traps everywhere which sometimes just makes your opponent want to sit still in one spot, and then you can use your boomerang blade thing to rack up lots of chip damage. She's.. odd.
Tron Bonne - Tron has tons of veyr large and damaging combos, and Lunchtime rush is still one of the easiest things to connect in the entire game. Her drill is just as good as ever and does good damage. Her gun which makes servbots stick to you is the most annoying thing, ever. Ever.
Viewtiful joe - ...lacking. He's annoyingly slow, but his Voomerangs are extremely pesky and a charged up one forces you to block for a fair while. His cannon super is one of the worst in the game though, it does absolutely pathetic damage, probably less than a freaking H move..
Zero - I'm sure everyone's seen his release videos where he has those 20 second long combos which end in an OTG super. He's insanely offensive and his shadow super pretty much forces you to block or die. Very mobile and hard to defend against.