I used to feel the Lati twins were far too broken for OU, but now I'm sort of leaning towards them being at least tested in OU. I think that the fact that those many common weaknesses is a real thorn in their side, not to mention there are some sweepers (Tyranitar for example) who can't be OHKO'd by their Attacks, and still be able to OHKO them.
As for Latios, a CB Tyranitar can come in on the CM version, take 62.87%74.26% from a +1 LO Timid Surf, and hit Latios with a Crunch for 175.50%-207.28% damage. Similarly, it can come in on Specs hits like Dragon Pulse, Draco Meteor, etc... and not be 2HKO'd (Draco Meteor has a very slight, slight chance to 2HKO, but it has to roll max damage twice in a row) and either Crunch it for the same damage, or just Pursuit it as it tries to flee away (OHKOing it of course)
Physical Latios won't be doing...much of anything to Skarmory. Base 90 Attack is pathetic by the hard hitting Dragon Dancers and such. Even a +6 Outrage from a Jolly Latios only does 67.37%-79.04%, meaning Skarm can just Roost off the damage, or just Whirlwind it away. It's not threatened in the slightest bit, unless said Latios is packing Thunderbolt.
Blissey remains the best counter to special oriented Latios, although, like a majority of people have said, Trick hinders her ability to wall him. The All Out Attacker is stopped by Tyranitar, Blissey, Cresselia, etc...
Latias seems a little more interesting, and perhaps a little more viable. It's support movepool is excellent, and it has superior defenses to Latios, especially in SpD. With a 252 HP/252 SpD spread, it can take quite a beating:
331 SpA Timid Yanmega's Bug Buzz vs. 364 HP/394 SpD Latias: 40.11%-47.80%
Only with Stealth Rock can one guarantee and OHKO, and Leftovers can fix that. The funny thing is, Latias can now use and enjoy Leftovers, meaning that it'll be even harder to take down. She can even take hits from things like Specsmence, as Dragon Pulse from a Max SpA Salamence with Choice Specs only does 79.12%-92.86% I know it may seem a little odd to post that, but the fact that Dragon Pulse can't OHKO and isn't even guaranteeing an OHKO with Stealth Rock support is scary if you ask me.
Latias also makes a pretty good answer to things like Tyraniboah, as Dark Pulse won't ever 2HKO. Although Tyranitar, namely the Choice Banded version, still gives the offensive versions plenty of trouble. Even a Max HP/Max Defense Latias takes 92.86%-110.44% from Choice Banded Crunch. However, Latias can happily put up Reflect, take 46.70%-56.04% from the same Crunch, and then PP stall it (although the Defense drop makes it very risky).
Latias can also take things such as Ice Beams and such very easily from the common Bulky Waters. 0 SpA Vaporeon's Ice Beam does a paltry 39.87%47.18% to 0 HP/0 SpD Latias.
Fighting resist is the icing on the cake, although repeated physical abuse hurts a lot. She'll always be 2HKO'd by +2 LO CC from Lucario, but she can set up Reflect before the second Close Combat, and start healing up (only can work with Max HP/Max Def).
I agree with Peachfuzz that a Choice Scarf Latios would be an excellent revenge killer to many of the metagame's threats (it can outspeed Deoxys-e!) Draco Meteor/Thunderbolt/Surf/HP Fire could work, although it's not meant to hit hard, just hit for super effective.
I doubt Psychic would be a good choice on many offensive sets of each, even though it's stronger than most of the attacks the twins have. The bad coverage, overwhelming amount of Steels that switch into it, as well as Tyranitar and Weavile who are happy to come in make it very risky.
