After using all of the first party contollers for the Xbox, PS2 and GameCube for a reasonable amount of time, I must say this is a real step-up for Microsoft.
The original Xbox controller was huge! Some people found it unconfortable, but I preferred it over the controller-s as the larger size made it more managable. With the 360's controller being even smaller, I was a little worried, but it's ergonomics are perfect (for me at least). Many hours of racing or FPS action will not leave you with aching fingers or thumbs. The triggers are sharp and precise, as are the analogue sticks. The face buttons are tactile and well placed while the bumpers (the sholder buttons that replaced the black and white buttons) are also very good. They are not designed to be used at the same time as the triggers, as games designers have realised, but holding a bumber button and its opposite trigger (LT/RB or RT/LB) is fine.
The center buttons are well placed as well, I have no trouble locating any of them without looking at the pad. My only niggle would be with the d-pad. While this is not a well used feature on modern games, it ca be a little difficult to tell if you've pressed the right direction some of the time. This is particularly evident while playing Perfect Dark Zero and trying to use the gadgets.
The wireless is awesome! Instant response and excellent range with a very good battery life. If you can get hold of some rechargable AA's and a charger, it's probably not worth the play and charge kit though. I've yet to run down the batteries yet, so I'm sticking with my AAs.
Don't jude the controller on any experiences you may have had ona shop display cabinet, the controller is far nicer to hold when it's free in your hand!
All in all, this controller verges on perfect. Well worth the extra money for the easy to use wireless as well.