Apathy is, of course, a hysterically funny book, and I can honestly say that I laughed at least once a page; Neilan's wit is razor sharp and, given that both the author and the protagonist have a pervasive (wait for it) apathy about essentially everything, his ability to keep every page fresh is little short of astounding.
Saying that the plot is thin is, to some extent, missing the point; the main idea behind the book is that nothing of any interest ever happens, ever, and so with a character who is fundamentally disinterested in ever doing anything, we shouldn't be surprised if it isn't filled with car chases and explosions (although death and sex play a fairly heavy part, even though Shane doesn't care about either of them). The charm of the book, aside from Shane's constant disaffected narration, is the quality of the characters. Words like "quirky" and "neurotic" get bandied about an awful lot in a positive and endearing sense, but every one of them is so deeply flawed that it's little wonder that everything collapses around Shane as he tries desperately to avoid making a connection with them.
Ultimately, however, the success of the book is to create a strong sense of empathy with Shane's situation. When we consider his surroundings of self-absorbed and self-deceiving people living in a world based on lies and desperation, Shane emerges as the only one with the right response to it. His constant rejection and disinterest, when viewed against the world he's expected to be a part of, isn't, in the end, really that surprising. If you think that's charming, then buy the book. If you don't, then buy it anyway, because it's nothing short of amazing.