As far as I can tell, the only reason this movie was made or even written, is that Seth Rogen wanted to play with guns. That is not to say it is at all a bad movie.
The characters are for the most part believable. Even the thugs are the kind of guys who one can't really imagine to have too many other employable skills, so as outrageous as they are, it kind of works.
As Mallory mentioned in her personal blog, James Franco shows himself to be an underrated actor. He is definitely better as a type of guy you probably know; ie. the gorgeous rebel/freak Daniel Desario in Freaks and Geeks
or the pothead dealer Saul Silver we see here.
Seth Rogen also does well as the fumbling guy caught between being a grown up (having a job) and an adolescent (dating a high school student). The two actors have a great chemistry, allowing the viewer to believe it when they profess their friendship status as bfff.
I've heard reports from some that it went on far too long. Having heard that before seeing it, I had no problem with the time. I can't say for long how I would have felt otherwise. I was still laughing until the end, so if nothing else I feel that if you're prepared for the length you should have no real problem with the movie.
One sentence review...
Pot + Guns = Hilarity, but go to the bathroom before the show.