It's difficult to not compare The Heartbreak Kid to Forgetting Sarah Marshall, and it doesn't do well in that comparison. The latter outperforms the former in nearly every possible way. Ben Stiller isn't at his funniest here, but he does manage to do ok in a few funny moments. Michelle Monaghan was the bright spot for me, she is a great young actress and handles romantic comedies just as well as she does any other genre of movie.
Ultimately, The Heartbreak Kid just isn't as funny as it should have been. There were only a few laugh out loud scenes, and there were long stretches of the movie that were just boring. The story was as predictable as possible, and none of the actors really seemed to be trying that hard. Rob Corddry, Carlos Mencia, Jerry Stiller, Danny McBride...they're all just doing the exact same bits that you've seen them do before. I have to admit that Mencia was kinda funny, though.
And as far as the R rating goes, it doesn't really add much to the comedy. There's some nudity, sex, and cursing, but none of that makes the movie any funnier/better. Sure, I'm not gonna complain about seeing Malin Akerman's goods, but if that's your reason for seeing this movie, you'd do better to track down her scenes from Entourage. You're guaranteed to have a better time watching those.