I think it'll be moderately successful. It's a great service with a wide selection and very fast (at the moment). But I don't really see any benefit of it over using any equivalent streaming service like last.fm, pandora and deezer. The ability to fast-forward, rewind and skip isn't that much to shout about. Plus there's no Linux client yet, which puts me at a disadvantage.
The only reason I find to use a service like this is when I don't have access to my own music collection, like at work (we are using spotify at the moment). But with ipods, squeezeboxes, fast connections and all that readily available it's not really a common problem. It needs to really shine in the recommendation, profiling and community areas to win over the competitors.
