No, I don’t get any money for posting this or referring anyone. Neither does my company. This is just to let you know my experiences with Safari in case you were thinking of subscribing.
Several of the books I’ve reviewed here lately I don’t actually own. My employer, CapTechVentures, gives every employee an account with the Safari Bookshelf by O’Reilly.
Now they certainly don’t expect us to read all of our books online, but view it more as being able to search a large number of technical books and be able to view the relevant material quickly (as opposed to Amazon, where you can search for something, but then you must buy the book). It is especially useful for all the technical work we do, and covers enough of both Java and .NET to be useful to everyone. Currently Safari has close to 2000 books.
So what publishers would you expect to find? Check out this diagram:

If you read a lot of books by these publishers, Safari could be a good investment for you.
What are the limitations? To view the full text of a given section of a book, it needs to be on your “bookshelf.” There are several bookshelf sizes. I use small, and that limits me to 5 concurrent books at a time. Once a book is on your bookshelf, it has to stay there for 30 days. Most books take up one slot, but some take a half-slot (0.5 slots) and some can take more than one, though I haven’t seen any books like that yet. You can upgrade your bookshelf size at any time, which costs more money of course.
That’s about it. I enjoy it as an employment perk. Would I buy it on my own? I’m not sure, since I can usually get my employer to foot the bill on any technical books I want to purchase. If that were to change, I probably would subscribe.