I love my Sony 600. the only problem I had with the software was using the synch feature, so I simply don't use that feature. Loading books on to my reader & adding them to the reader library is easy to do.
I can get multiformat LRFs, epubs, pdfs, and even convert my MS Word docs to rtfs to upload to my library & reader.
Sony doesn't lock you into just purchasing from their store, you are free to go to any store that has epubs & pdfs that utilize Adobe Digital Editions.
For me the Sony gave me the greatest number of choices & flexibility, that's why I got one.