I have both an Amazon Kindle and an iPad, and I read books on both. The kindle is superb for normal text books but sucks rendering PDFs so I leave magazines and technical books for the iPad. But in both cases I hate to have content with DRM that I cannot freely transfer between both and make a backup of.
I don't care that both Apple and Amazon are big companies. Amazon has removed books legaly bought. Apple is the new dictator and locks you more and more into only their ecosystem. I want to not just being able to re-download my books, but store them wherever I like. Nothing guarantees you that in the future they won't decide to shutdown services (like EA does with videogames) or remove content. When you buy a paper book, if later the publisher has licensing issues does somebody come to your house and retrieve the book you bought?
I don't care about watermarked PDFs stating who bought them. It works because if you share them you can get identified and prosecuted, but it doesn't limits where do you read them.
The same that I have dumped all my old PC games into ISOs, all my DVDs into divx/mkv and all my CDs into MP3s, I wanted to have an easy way to deploy the books I've bought in any of my devices, so after some digging I made this small list of required software to "free your books".
You just need four tools, and all of them work at least with Windows and Mac.
It is not uncommon to see "no DRM found" messages for example in Requiem, as for example some free iTunes books are simply in epub format (well, and those in iBooks are mostly epub too).
One final warning: While this will remove the protection, it won't remove all authoring nor owner details, so don't try to be too smart and publish/share your books. This tools are meant to help freeing the format, not "cleaning for piracy".