A lot of people keep bringing up the issue of quality. Personally, I think that should be taken as a given - the assumption is, it's good enough that you would buy it if it wasn't online for free, so would being online change your buy/not buy?
For me, at least, it would. I'm more likely to buy something I already know I like, particularly if it comes with extras like art, good formatting, an editor, good binding, etc.
Steve Jackson Games (who produce GURPS) playtests most of their books online (for subscribers to the Pyramid webzine). I would never have given Transhuman Space a second look, fearing a subpar execution, if I hadn't been able to read through it online and discover that it was a thing of beauty. As it is, I try to own practically all of the Transhuman Space products.