Make no bones about it. There are a lot of things wrong with this movie. It's clichéd the whole way, not very funny, predictable, and illogical. Let's start at the beginning: characters. There's the boring, luckless guy - giving Stiller another notch in his boring, luckless guy belt - the allegedly wild, but in reality just fairly normal, love interest - whom Aniston plays well, but really needed no effort to do so - the fat, jovial friend, and then the assortment of clichés: an annoying daredevil Australian guy, a confident Spanish guy, etc. The storyline: the beginning is slightly unusual, but thereafter goes into the standard any-movie style, with every plot turn as predictable as your average knock-knock joke. The biggest problem was that Stiller's character's "development" really seemed to come from nothing - like your average school play, the writers knew where he started and where he ended, but didn't put enough stock into properly telling the middle bit. Finally, the alleged 'jokes' were nothing but highly watered down versions of standard gross-out humour; there was a regulation chunder scene, sweaty fat men, etc.
In conclusion, the simple fact about this movie is that learning the meaning of the word 'shart' was the only good thing. Hamburg really dished up a dog's dinner here, and the sugar coating of Stiller and Aniston may have lured the viewers, but the taste left at the end was just as rancid.
Final comment: This film may have been dreadful, but Aniston still picked a better between-Friends-seasons movie than Kudrow's odious 'Marci X.'