Based on some of the info in Martha Moody's online bio, I infer that that this book has certain autobiographical elements; like Clare, Moody is (was) a physician, and she has had the same best friend since college. I hope there aren't too many similarities other than that - if you choose to read the novel, you'll see what I mean, but I don't want to give any spoilers.
For the most part, I found this book rather irritating. The writing was pedestrian and I didn't like the characters very much, but the story itself was interesting enough to keep me reading - and that's the annoying part. If it had been duller or more offensive, I could have bailed on it more easily, but I wanted to see how it turned out; and frankly, I kept hoping it would get better and I would like it more. However, it generally didn't resonate with me emotionally, and I didn't get much of a sense that the characters grew or developed; life just kept happening to them, and they kept making questionable choices. The parts I liked best actually had little to do with Clare and Sally's friendship; I was more interested in Clare's medical practice as an AIDS specialist in Ohio, and I sometimes got the sense that Clare was more interested in Sally's Southern California home and life than she was in Sally herself (which was reinforced by Clare's initial reaction to Sally's move to Idaho).
Best Friends seems to be a pretty popular novel, so it clearly appeals to a lot of people, but it just didn't click with me. I think women's long-term friendships can be great frameworks for novels, but to cite one example from my recent reading, I thought it was done better here.
I read this for my book club. I suspect I may have a minority opinion of this one, but no one likes everything - and just because it wasn't my thing doesn't mean someone else won't love it.