Forgetting Sarah Marshall was a disappointment. It wasn't as funny as its partners (40 Year Old Virgin, Knocked Up, Superbad) and had a lot more raunchy stuff than I was willing to put up with, which is saying something when you consider that I liked the others. Maybe that sounds a bit stupid, but that was how I came out of it.
The story is fine. It's a break-up story, and Segal has written some unique and honest perspectives into it. While the overtones are conventional, it has good story structure and follows an arc.
Some of the characters feel a bit abandoned, though. We get a good conclusion for the Peter and Rachel, and the jerky-yet-somehow-sympathetic Snow, but Sarah Marshall gets dumped by the characters and the story, and even some of the supporting characters the movie invested time in are simply left out. Jonah Hill's Matthew the Waiter is present in most of the movie as a star-struck struggling musician who gives Snow his demo, and the last time we see him is when Snow blows him off. And that's it. He's mostly there for laughs, but it felt like we got to see too much of a character for him to be left there. Jack McBrayer's Darald the newlywed is also around for the whole movie, relating to the main characters his sexual struggles with his new wife. When a movie actually cuts away to detail a character's problems, you expect them to have some meaning and find a conclusion. But apparently they were only there to pad out the laughs until we didn't need them anymore and could conclude the main story. The other Apatow movies usually have characters like this as well. Weird and funny side characters who only appear for a short time, and occasionally reappear at opportune times, but Matthew and Darald had too much invested in them to be left off (mostly Darald -- we only saw exactly what Matthew did in front of the main characters, so I'd accept him as casual casualty).
A pleasant surprise was Mila Kunis, because before Forgetting Sarah Marshall, I only knew her from That 70s Show and Family Guy, and she seemed capable of only one style of delivery. But here she shows she is indeed a genuine actress, capable of creating a sympathetic and believable character with natural delivery. Her performance in That 70s Show always came off very shallow and limited, and I assumed that was due to her abilities and not the character. Apologies, Mila!