Yeah, the app won't show up in the store if you're still on iOS 5 or lower. So there's that.
The web app is certainly interesting, but I don't see a lot of point to it - I already have Rdio on the iPhone and iPad, and the Mac app or web app. I can definitely see how it could help for music discovery - it adds an extra layer.
It's an interesting concept, and the interface is pretty cool, but I just don't see it as something I'd find useful in the least. But I'd reckon most people on this site would say that, considering we're all pretty adept at finding music on our own.