One more thought about EMusic, specifically using EMusic to build a music collection for DJing. This is an avuncular scolding for would-be DJs out there, maybe with a dash of "do what I say, not what I do" thrown in.
So as I said in my earlier post, EMusic is a good way to ramp up a collection if you are just starting out or looking to see what the view is like from behind the DJ table. And it's also a good way to branch out and discover new music.
I think it is a bad idea to be an exclusively EMusic DJ, especially if you are using the (less than stellar) social recommendation aspects of EMusic. If you do, you're going to end up with exactly the same collection as all the other DJs that use it and we would all start to sound homogeneous and boring. It's lesson one of Web 2.0: crowd-sourcing can easily breed group-think.
On its own, EMusic is not a very good tool for learning about music. Sure it is a source for music, but without other resources to learn about musicians and jazz, it's not a good way to actually learn. You can easily blow hours browsing around at random, listening to hundreds of 20 second clips, and not gain anything by it. But if you have some other way of learning about musicians, bands, composers, jazz culture, that sort of thing, then EMusic is a useful tool to have access to.
What are those other ways of learning? Friends, books, maybe the Internet (Swing DJs, Pandora), and of course cd/LP liner notes.