I saw this today and I was hugely impressed. I should state that I've always been a Scorsese fan, but even taking that into account this was undeniably pure entertainment. The pacing, performances and script were spot-on. I laughed a lot during the movie, there's some really funny stuff in there - and yes I did laugh at the elevator shootout and I think that was intended by the filmmakers. There's a certain point at which that scene becomes absurd, but in a good way. It's acknowledged slightly in Damon's performance, as he pauses for a moment almost in disbelief trying to comprehend everything that's happened in the past few seconds.
The violence is often laced with that kind of black humour - such as when Costigan injures his hand while beating up the Italian mobsters, or the guy upset that he didn't go into shock after being shot... but on the other end of the scale there's some proper down and dirty stuff that really hits hard. I liked that the film was able to mix those tones together and stay true.
Alec Baldwin stole every single scene he appeared in. I'm really liking his tendency to come into a film with a strong suporting part, blow people out of the water and move on. Having said that I think that everyone involved gave superb performances. Nicholson was reigned in and DiCaprio (who I'm a fan of anyway) is confidently moving ever away from his teen heartthrob image. Still looking a bit cherubic (oh, what a curse!) but I never for one second failed to believe in him as the character.
I've seen Infernal Affairs so obviously most of the plot wasn't news to me, but I for one did like the slightly altered ending. It did come a little as a surprise as I'd completely forgotten about the character involved but it was a good payoff for the whole thing.
My film of the year by far.