I must admit I skimmed the post the first time :). I too have run a book group for the last few years, and we've toyed with this idea several times. The advantages are clear in terms of not having to hit a tough deadline, and being able to tackle longer books. I think it can work with anthologies of short stories and essays, but for longer narrative works I wonder about the following (perhaps you can provide some insight):

- Will new people be willing to join in the middle of a book?

- If people don't like the book, do they just have to stick around for the next N meetings? If not, does it lose its place in their routine causing them to forget about it?

I think these concerns may be specific to more "open-access" book groups that anyone can join, rather than long-running groups of existing friends.