I'm pretty impressed with the service too, and I really hope Amazon can get some of the labels on board with the non-DRM. Unfortunately a quick search of some of my favorite artists yielded few results. Of the four bands I searched for, I only found albums for one. Granted, I was looking for some obscure ones, but they're really going to have to work hard to add artists to their store.
Personally, I won't be buying music from any of these stores. I rarely buy any brand new releases these days, and the everything else I've gotten off of the Amazon Marketplace. I can get a CD that's still in very good shape for less than $5, and even with shipping I come out ahead of buying an album at any online store. Plus, I have the physical CD, and I can rip it however I choose (FLAC, these days). There's just something about having the liner art...
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Matt