I got a free subscription to Rhapsody Radio Plus (which is their basic-level service) which does something sort of like this. I can tell it that I like a band, and it'll play a stream that contains songs by that band and other similar artists.

The real problem with that is that the concept of similar is pretty hard to define and is always implemented poorly. Just because I like Pearl Jam doesn't mean that I like Creed in any way. Quality (which is subjective anyway) seems never to be taken into account in such algorithms.