Just because I am an incorrigible nitpicker, I have to point out that correctly, that's converting from Mobipocket (Amazon) to ePub (Sony) -- Amazon and Sony are vendors, not formats. Mobipocket and ePub are the formats, and .mobi and .epub are the file extensions. Yes, I know: those who live by the nit shall die by the nit, and there's a giant killer nit waiting in my future somewhere.
Part of the problem with ebook readers that are tied to specific stores, right now, is that nobody is quite sure how the publishers' cartel ("Agency 5") which has been formed is going to work out in the long run. Right now, they're forcing retailers to charge more for ebooks than for paperbacks -- sometimes even more than for hardcovers -- and they clearly want the prices to go up, even though their costs, compared to a physical book, are considerably lower. So I'd be
really paranoid about buying an ebook reader right now with the intention of depending on any given manufacturer's bookstore.
If I were buying a reading device today, I'd be looking at flexibility. What file formats can it handle? Can I get the books on there without outside intervention? How much control does the manufacturer have over my device (e.g., the 1984 debacle)? Can I read what I want to read -- and perhaps more important, will I still be able to read it next week, even if the vendor changes formats, goes bankrupt, or just abandons the market?
My personal solution to those problems has been straightforward: I do not buy DRM-restricted ebooks, and I don't buy ebooks that cost more than their pbook counterparts. I realize that some people would rather trade rights for convenience, and some are wealthy enough not to care about ebook prices, but even for them, the matter of flexibility, store independence, and future-proofing should be a consideration. Platform lock-in is only good for manufacturers and publishers; it's very, very bad for us.