Hot Tub Time Machine is the kid brother to The Hangover. It very much wants to be like its older sibling, but it just isn't there.
I thought The Hangover was incredibly funny, hitting almost all of its jokes. HTTM, on the other hand, has some that flop badly, and some scenes that need a lot more of them.
That said, HTTM has what might be one of the funniest running gags in a movie. I could have watched Crispin Glover as the bellhop for the whole of the movie.