I think a lot depends upon your own personal reading habits, and the definition of 'reference' . I've built up my paper O'Reilly collection over the years (up to about 35 now), in addition to my 'digital collection' - WC3 specs, perldocs, online references etc. - and I tend to find that (with the exception of the camel book and the web-reference one, both of which are so well-thumbed that I can find anything I need from there faster than I can on-screen), I use the digital sources on a day-to-day basis because it's generally a 'quick reference' that I need when I'm working ("how do you capitalise 'selectedItem'" or "what order are those parameters in again?") , whereas the more 'fun' books (Mastering Algorithms etc.) are read in the bath and pondered upon and left fermenting in the mind until the next project - something that is more difficult to do with online copies.
Certainly, if reading a screen is your thing (and one of my colleagues who has a subscription is quite happy with it - sits and reads all evening ) then safari is a great thing (and from a great publisher - I *do* like those bookshops that have an exclusive O'R. shelf ), but I personally would prefer to have paper copies of the 'essentials', and then wait for the others to turn up in Oxfam or on birthdays :)
Cheers,
Ben