Returning to my reviews of online music sources, let me say a few words about Amazon mp3. This source has two advantages over iTunes: it's slightly cheaper (by about 10 cents per track, $1 or more per album) and ALL of the files are high bit-rate mp3s. Those are both big reasons to shop Amazon.
In terms of the catalog of available music, it seems to be mostly the same as iTunes. Lots of the same obscure tracks are available through both, and I have yet to find a track on one but not the other. (Of course, sometimes neither source has what I need...)
The integrated search engine is good (it's integrated with the rest of Amazon), but the browsing interface is less than stellar. iTunes still has the edge there. My only other complaint with Amazon is that there are very few reviews integrated in with the browsing interface. Not that I even put much stock in them, but it is nice to have something to read if you've stumbled across a new artist.