Some spoilers in the post below, so you know, be careful.
I enjoyed this movie, but thinking about it now, a day later, I have this nagging feeling that it wasn't as good as could have or should have been. I'm actually struggling to think of any really clever, memorably funny lines (you know, the way that most people can call Simpsons jokes to memory verbatim). RDJ's line about going "full retard" is the only one I can think of.
Steve Coogan is a great comedy talent but he's not really used at all, he's basically a straight man and his character's only funny moment is when he steps on the landmine, which seems like a waste of a great performer. Similarly Tom Cruise's character seems like it should have been funnier than it was - it's funny because we know it's Tom Cruise and contextually it's amusing to watch such a self-important, vain weirdo get all dressed up in a fat suit and swear viciously, but take Tom Cruise out of the equation and the character really isn't particularly all that interesting. Presumably the fact that the drug cartel is led by a 12-year old is intended to be funny, but I didn't really get any sense of the character, why he's their leader or so feared, which again seems like a missed opportunity. Similarly, once it's revealed (fairly early) that Nick Nolte's character is a fraud, his character just stops dead, there's no further development.
That said, I did like it, I thought it was nice to see a solid story-driven comedy and there were enough decent laughs in it to make it worthwhile. Robert Downey Jr is predictably the highlight - my very favourite moment in the entire movie is when Lazarus/Osiris yells "come on man, spit that shit out!" and there's a brief moment when a smile creeps across his face before he snaps back into character. I don't know if it was a scripted moment and meant to be Kirk Lazarus being so self-satisfied at his delivery of the "line" that he momentarily broke character, or if it was a flubbed take with Robert Downey Jr actually making himself laugh and they just decided to keep it in the movie, but it was a moment of genius and I laughed the hardest at that.
But yeah, I can't shake the feeling that while it's an enjoyable enough movie, if they'd just pushed it a little further, it would be a great comedy classic.