Michael Blowhard, today, has his take on American Splendor. He is much less enthusiastic than Futurballa, but he does give links to David Edelstein's more appreciative review, along with Terry Teachout's take, and to Harvey's own site/blog.
I think the main point that Michael misses and that Terry Teachout sums up so well, is that the film is not about Harvey Pekar. Paul Giamatti plays the character in the American Splendor comic books, with scenes taken straight from the books and even framed in comic book style. Certainly Pekar writes about himself in his comix, but it is a fictionalized self. Perhaps it would have been interesting to get a biopic about the real Harvey, but it would be a very different movie. Teachout compares the movie to Terry Zwigoff's Ghost World (also a great flick). I think if this had been a more realistic movie the comparison would be with another Zwigoff film, Crumb.