The negative reviews here are most likely from guys for whom this movie struck a nerve. The acting and writing are excellent. You know this because this movie actually causes the viewer to feel compassion for and root for a character who is very nearly a complete solipsistic, self-absorbed buffoon. I don't know how someone could view it and find no comedy in it, unless they so strongly identify with the main character that they too acutely feel his pain. I was chuckling throughout and the perfectly delivered line "You wish!" made me lose it.
A big part of the humor is that the main character is a devotee of what is itself even a fading form of nerddom: the Dungeons and Dragons inspired tabletop fantasy role-playing game, and that he's even been able to assemble a clique of players for his own homemade, unpublished specimen. It's one of those domains in which the competition is so fierce because the stakes are so small.
The movie reminded me a little of the also excellent Big Fan (2009), another humorous character study of an obsessed man-child.
I was surprised, after watching it, to see how old this movie was. The actors in it, esp. The lead, deserve more additional roles and recognition than they've gotten.