Observe and Report is not a good comedy, it's just not that funny although it has some really funny moments. It's kinda combination of Mall Cop, which is okay, and other comedy movies.

another one is Zack and Miri Make a Porno. this one is hilarious but you better watch it alone or with friends - not family.

both these movies feature Seth Rogen who is sometimes very funny.

Fired Up is a good comedy for young people and although it has the usual stuff for a comedy for young people, it's still very funny.

I just saw Burn After Reading a few days ago, I know it's an old one but I really had high hopes for it; hopes that were not met!
It was really disappointing

I heard that "I Love You, Man!" is a nice one, haven't seen it myself.