I took a testing account from the Safari Bookshelf web services provided by O'Reilly Media. My physical bookshelf is largely composed of Computer Sience and IT-related topics books. So It was quite consistent to take the 14-days evaluation from them but I'm still hesitating to continue with the paid subscription (around 20 dollars each month). It's not really a matter a price as my budget for books is clearly higher than that… but the main issue is regarding the freedom of the user to use the book. You must always be connected in order to read a book. You have the ability to buy token for downloading chapter but this is very limited and it's very easy to use all the month token for 4 or 5 chapters of the same book. Another issue (at least for me), You can't easily store the book localy for offline reading or local indexing (the proposed search via the web services is not very powerful compares a local hyperestraier, Mnogosearch or Lucene). The most strange thing for O'Reilly media (known for publishing a lot of books in the free software world) is to use a proprietary software for safari built by Bureau Van Dijk and using a very basic web interface. Ok, they didn't take a fully DRMized approach for the service but … I would expected a more free approach for the personal use of the book read. Until now, I can't make my mind for the subscription.
Update: I resigned from the Safari subscription. Not being able to save a local copy is really a major issue to me. I'll come back to such services when I'll have the same capabilities of a paper book. Like being able to store it locally, keep it or share it with a friend.
Update2 - 2006-09-10 : Beside Safari Bookshelf, O'Reilly now offers the ability to buy a PDF version of their books. I bought one to test the service and I was very pleased (compared to Safari Bookshelf) that you receive a standard PDF file without encryption. Congratulation… a good move and it's really more flexible than Safari.
Update3 - 2006-11-26 : I bought a second PDF version of a pocket reference from O'Reilly. Still very nice (standard PDF, no encryption, indexable) and now they offer the ability to get an announce when there is a revised version. The current offer for the pocket references is half the price of the physical version… I hope that O'Reilly will offer more and more books in that service instead of Safari.