As someone whose tastes are rather esoteric, I love eMusic, the only problem is the per track downloading system. It works quite well for your standard 14 track album, but if you want to buy an album such as J Dilla's Donuts (which is 30-odd tracks), you end up having to use your entire allowance on one album. Sucks, but other than that minor gripe, it's a good service...
eMusic: Digital Music As It Should Be