Another Porii Sames rant:
As most of you know, I've been an avid Dream World player, previously on Smogon and PBC (i still play beta), for over a year now, and I will say that Gene is one of the most ferocious threats out there.
Scizor is currently (or was previously, i havent checked up on that as much now) the most used Pokemon, but try as it might, Scizor isn't breaking through certain walls anytime soon. Skarm is always going to give it troubles, as is bulky Volcarona, Protect Tran, Zapdos, etc. That being said, Scizor is still a very good poke because it can utilize U-turn to at least dent a few of its counters and not lose any HP itself.
Think about a Poke that retains a strong U-turn, more speed than Scizor, an AMAZING Special movepool, great Special Attack, usable defenses, and even some ways to take down its counters?
Genesect is not something that will completely remedy all of your weaknesses on a team. Genesect is not just a replacement for Scizor; they both have their own uses. Genesect is a monster that has absolutely fantastic synergy with just about anything and if you predict right it literally can threaten 90% of the OU meta. Rachi seems a safe switch in if Gene has the Attack boost, but a U-turn followed by a Flamethrower later immediately stops this.
Even Heatran is never 100% safe vs. Gene. I very seriously laddered with an Earth Gem HP Ground Gene and it broke 1550 and got me into the top 20 on Beta, which isn't groundbreaking but it goes to show that it is clearly viable.
Since Gene can U-turn and can come in on SO many pokes so easily, it can be an entire Voltturn core by itself, being able to hit hard physically and specially while also being a reasonably fast scarfer.
Almost all of Genesect's counters are promptly handled by a Sub Liechi Dugtrio. Rachi, while not OHKO'd by EQ if it is 252/0, takes about 80-90% and then, if it hasn't Wished up (assuming Gene has U-turned on it), it can either be KO'd by Dug alone or used as another target for Gene's U-turn.
I don't even need to talk about Heatran; Air Balloon is shattered by a U-turn and even if it isnt, 200 BP Reversal does a chunk (iirc it doesnt OHKO).
Tenta also takes a huge amount from the Dugtrio assault, whlie Toxicroak(Subbing vs. Sucker Punch is a fantastic advantage) is flat out KO'd. +1 Reversal also OHKOs Blissey and Chansey alike (Chans has a 'chance' to live but it will again be used as something else for Gene to gain momentum against), and Dug can do a large amount of damage to Volcarona as well, beating it if Rocks are down.
And this is ONLY Scarf Sect. I'm not even considering the odds of it being Sash, Expert Belt, or Rock Polish, all of which are great sets in their own rights. RP is one of THE BEST set up sweeper sets in the meta (or at least the DW meta...) and teams without Chansey or a Fire are always at risk from it.
Genesect can also surprise you out of nowhere. You can bring in Tentacruel after switching out your Specs Latios and it can Thunderbolt you and just switch next turn. A Special Attack Download Gene could turn from an easily handled Scarfed to an absolute monster in one turn with a Rock Polish. You could fodder your Gyarados as Gene attempts to Flamethrower and then bring in Tran only to take an HP Ground, or have it be Sashed and completely surprise your supposed "counter". And you never have any idea what exactly it's going to do when it comes in.
Gene is an absolute MENACE and is, in my opinion, THE best Poke in DW, and that was Pre BW2 and Post BW2. It is NOT to be underestimated and it will surely be a force to be reckoned with in OU.