I've never actually bought a book from the Sony store. When I first got my device, BooksonBoard sold the books I wanted for much less, and even since the implementation of agency pricing, I tend to buy from them as their interface is easier to use.
I don't really understand the whole I've-got-a-Sony-so-I-must-buy-from-the-Sony-store line of thought. There are tons of other places that sell books with Adobe DRM that work just fine on the Sony! (And are often easier to use, and occasionally cheaper to boot!)