There are Spotify fans here. There was a pretty big thread when it first became available in the US. I wasn't impressed enough with the desktop app to switch from MOG. I don't really use MOG to discover new music, but to audition music that I discover in other ways. While it can stream, I don't like it's streaming service. I imagine I'd use Spotify the same way. I'm curious how you use it to discover something new.
I occasionally use Pandora's streaming to discover new music. It's great for exploring a genre you don't know, but not so much for a genre you know well and want to discover something new in. It gets pretty repetitive.