It Depends on the types of games.
At Halo 3, I'd say I was good. I know the maps well and all the spawns, I can go on a kill streak fairly easily and I kick ass with the Spartan Laser.
On things like Modern Warfare 2 though, I'm not that great, the guns just don't feel right to me.
RPG's: I'm good at, I know how most RPGS are and can go through them a few times, the only problem is that some times I get underleveled, which is why I never completed Final Fantasy 3 on the DS.
RTS' I'm not bad at, I can counter well if I know the enemy properly, I use Scouts to my advantage and know how to give out and collect supplies, etc.
Sports games, I have no interest in.
Racing Games: I was brought up with Crash Team Racing, which I played to death. I do well in online Mario Kart as well.
Realistic Racing games, no, I just can't grasp them, I suck at them.
Fighting games, no again, unless it's simple like Super Smash Bros. Melee/Brawl.
I only play 1 MMORPG, which is WoW, so I'll talk on that. I'm a Hunter, and I'm fairly good, Arena Rating is 2400 and I'm in a solid raiding guild.