The disc based system MP3.com came up with was pretty clever, but it also opened them up to an easy loss in court. Basically they lost due to not actually playing back the users version of the song, and instead playing back a single copy that MP3.com didn't have the rights to rebroadcast.

As for the actual service, I'm going to give it a look probably this weekend. The downside is yet another service using up data to mobile devices at a time when the carriers are really trying to tighten down with transfer caps. I wonder how much longer it will be before the tech companies just band together to build a better internet to allow them to keep moving forward. Google is already on this path with the gigabit experiment.