I started using Calibre earlier this year when I was looking for a good cataloguing program for my reviews. I've been using it since '06 or so to manage my ebook collection, but never for cataloguing purposes.
But it has been growing in utility and I just found out that in the newest release [2.67] that the creator, Kovid Goyle, has added the functionality of half stars for rating. It has always had a 5 star rating column, but now that it has the half stars I'll be going through and using that for all my books instead of just leaving it blank and looking at my review.
I love it when a program fits your needs perfectly.