The only one I've looked into is the Nook, but it seems to me like it is fundamentally flawed. It has a lot of bits so it can't be brute forced. But, they use a pass phrase to generate the key. The phrase is your name and credit card number, so it's not as many bits. But if I get a hold of your Nook I can get your name from one of your screens. And the last four digits of your credit card. The first 6 digits of your card are not secret either and are determined by your bank and card company. If you don't know it at least many can be eliminated. So the only really secret part is the middle section of 6 numeric digits. I'm not sure how long it would take to brute force but it doesn't seem like it would be too long and it's easily parallelizable. So if you leave your Nook somewhere not only could I copy your books, I can have your credit card and name as well.