Price, not convenience, is the hurdle. Until POD's costs come down, it won't be a viable option for book buyers. Why pay $18 for a book they can have for $10, already in print on the shelf, or just two days away from online stores?
For real convenience, why not an eBook? I just bought a bunch at $9.95 each and had access to them instantly. If you really MUST HAVE them right now, eBooks are the way to go.
What POD could do, though, is keep "out of print" books in circulation in the market. Old classics, public domain stuff, hard to come by reference books, text books, etc. So you have a catalogue of literally millions of books without the overhead of printing, distribution and shelf space. Now, that would be a great service.