I saw this movie about a year ago, and found it to be completely Laugh-Out-Loud funny. A real winner, in my mind. It had the underlining of a stupid comedy, but indeed had an actual plot as well. Much like the hit comedy, Elf, in fact. It had a few serious moments, sandwiched between hilarity. When Stiller shot Black's horse, an ordinarily sober moment, you found yourself laughing at his bumbling antics. You can actually find common ground there, as I'm sure many people have done something they wouldn't want a friend to find out about, lest your friendship should end.
When I found out how much people disliked this movie, I was completely dumbfounded. That absolutely terrible movies like Napoleon Dynamite had ratings higher than it makes me wonder about the sanity of people on IMDb. Take my advice and rent it. If you don't find yourself laughing at least once, I'll compensate you the rental fee. =D