Definitely on the darker side of the comedy spectrum. I personally enjoyed it and found Seth Rogen to be a very sympathetic anti-hero. Many others who detested the film think that he's horribly unlikable and despicable. I feel like, while exaggerated, the portrayal of Bipolar Disorder was fairly accurate. There were several scenes involving delusions which were scarily accurate.
There were certainly laughs to be had, but this film is pretty bleak. Many are all up-in-arms about a purported "rape scene," but that's not what I gathered was really going on at all.
Anyway, I'd give it about a 6.5 / 10.0. I certainly did care about the protagonist, was able to laugh at the hideousness and absurdity of the characters in the film, but the plot itself left something to be desired.