Unless Amazon have done a deal with the Music Labels, you'll still only have access to your own music on Amazon's cloud. So if you want 20GB of music up there, enjoy uploading all 20GB of it. Unlike Apple where you only need to have your itunes list sync with what's in the itunes servers and within (I would guess) less than 1 hour you can be listening to anything you own on any of your iDevices.
To me this is a half-hearted "me too" attempt. Without Music Label support (to stop the huge uploads via allowing music streaming), access to the service on other Safari mobile browsers (the press release says this "To access Cloud Player for iPad, customers simply open their Safari browser"), and something equivalent to iTunes Match this whole service is just not as good as the Apple offering.
Add in all the App syncing and other stuff the Apple version has and you can really see how the Amazon version falls short. If I was to choose between the two services, Apple's and Amazon's, Apple's would win everytime. The Amazon service would really need to get much better before I'd ever consider it. (That's if I was a US citizen and in a place able to consider both, but I'm not from the US, still I can have an opinion on these things).
