I use iBooks but like a few already posted I never buy books from Apple. I usually get my books from Sony or Amazon, then strip the DRM and convert if necessary.

So far iBooks has the best combination of features for my tastes, though it is rather lacking, which doesn't say much for the others. I like the two-column landscape and good dictionary support. Bluefire renders text better but lacks good dictionary support and two-column landscape. None of the reader apps has enough options as far as formatting. I would gladly pay $10 for a great reader app if it combined the best features of all the apps currently available plus a few dozen fonts.