I visited the Amazon MP3 site last night and thought it was OK. It doesn't have the seamless interface that iTunes has, but not too shabby for a site that's widely considered to be "beta". I do like the flash audio preview feature.
One thing that I did read about the site last night was that all purchases have a digital stamp put on them indicating various information, including who purchased it through the one-click store. This is to help prevent piracy on file sharing sites.
I still think that this site can't really hurt iTunes all that much. A bit of competition never hurt anyone. Besides, you can still play all the songs that are purchased on any iPod or iPhone. There's no harm in that.
Erik