I don't know much about Grooveshark (or Rdio) but one of the early selling points of Spotify is that it uses P2P - the first minute or so of a song is streamed from their servers but the rest is P2Ped from other users. Which means you very very rarely get buffering (apart from today :-/)
Don't think I've ever experienced buffering with a modern connection on any of the services I've reviewed (MOG, Rdio, Rhapsody, Grooveshark, etc). It's really just a cost saving mechanism for them more than an anti-buffering agent for us.
