"Serial" is a fairly good movie that instead could have been great. From the very beginning of the movie and the sappy and straightforward opening theme song, it's clear that the director opted not to capture the subtleties and sharp satire of Cyra McFadden's brilliant novel, but rather to paint a much broader and sitcom-ish picture.
From a commercial standpoint, this wasn't necessarily a bad decision (though the film wasn't much of a hit): the novel was very much about a specific place, Marin County, with tons of inside references, and a movie as slavish with the insider details as the novel was wouldn't have played very well at all outside of the Bay Area.
Still, the end result is disappointing. Not as disappointing as, say, the movie version of "Bonfire of the Vanities," but nonetheless along the same lines: a satirical page-turner brought to the screen without its sharpness and full potential. The dialogue feels forced at times, and we never get a clear enough sense of the characters to invest our full interest in them. I'd argue that some of the casting and direction was off the mark: Bill Macy, as great as he was on "Maude," isn't right for this film, for example.
As someone who was a young child in late-1970s Marin County, I wish there were a film that truly captures the quirkiness (some would say craziness) of that place and time. "Serial" tries, but doesn't quite get it right.
As a side note: Although Cyra McFadden's novel "The Serial" is out of print in the US, there's a pretty-recently-reissued UK edition that contains a really interesting forward from McFadden. Diehard fans of the book should check it out. Easier to order than you might think, through amazon.co.uk.