I have been a subscriber to this service for several months and I think it's getting better as it goes on. Originally the number of books was limited and some of the bestsellers (like Programming Perl) were not available. That has definitely been improving and with the addition of other publishers looks like becoming a very useful resource.

Personally, I like books like nutshells in paper format - much quicker to browse while at the code face and also easier to transport for those laptop moments. However, the bigger books are pretty usable online and save lugging a whole zoo around with you. I find myself switching books every month and exploring more technologies than I otherwise would - very useful. Of course there are some permanent reads on my bookshelf and when one gets too useful I find myself spending the money.

Overall, I've found it useful as more of a research tool. The dead tree nutshells seem to be the mainstay of my paper collection for day to day development.