One of the more endearing things about Forgetting Sarah Marshall is that it's widely known to be a personal script from Segel and semi-autobiographical, yet it doesn't pull any punches with Segel's character Peter Bretter. This could have been a standard R-rated breakup comedy, but the nuance that was put into the Peter character elevates the movie a lot.
Peter is our protagonist but he's also a flawed character dealing with a breakup. But not in the sense that he is pining for his ex, necessarily. He's hurting from the breakup, but it's also confirmation for him that the rut he's in is not healthy. There is one scene in particular where he confronts Sarah about their relationship and he has the upper hand at the start. But the conversation quickly changes to Sarah's perspective of the relationship and it gives us a lot more insight than I would expect from a standard R-rated comedy.
But even if we cast aside the relationship stuff and personal healing aspect of the story, Forgetting Sarah Marshall is a freaking hysterical comedy that has a phenomenal group of supporting players that are used the perfect amount and always leave a positive impression.