I went to see The Hangover as a throw-away evening, and was pleasantly surprised.
The movie was funny, fairly intact on the narrative side, and had some laugh-out-loud moments.
My only criticism is that while good, actors Ed Helms, Zach Galifianakis, and Rob Riggle didn't feel like they were acting that much -- or at least acting out of the type they've nailed very well. Maybe Helms was.
The same may be true for Bradley Cooper, but I'm sadly not familiar with him. I do now want to catch up on more of his stuff (and am now looking forward to his role in the A-Team reboot), thanks to this film.