It really depends on what you're looking for. If you have a PC capable of playing current games and you like to game on PC as well, then it's worth noting that some 360 titles like Left 4 Dead and Mass Effect show up on PC as well. If you don't game on PC or you don't want to upgrade then by all means get a 360. The new systems have vastly improved failure rates and the exclusives are great.
Most of the good exclusives have already been listed by other members, particularly Halo, Gears, Left 4 Dead, Mass Effect, Crackdown 2, Splinter Cell Conviction, Dead Rising could be hit or miss, Ninety Nine Nights 2 could also be hit or miss, the upcoming Fable III... plenty of good exclusives coming for both systems, so it really all depends on whether you game on your PC or want to go that route.
Personally I recommend the 360, but if you game on PC and have a PS3 already then you're only going to get a few exclusives that you can't play elsewhere. Such is Microsoft's business model.