My guess is that my usage, minus downloading on bittorrent, is about 12gb/month. Maybe more, since I don't really have a good handle on how much Netflix streaming over PS3 really uses.
Including torrents and Netflix my guess is it's around 25gb/month.
I tried the demo on my PS3, and I was dis-satisfied with the service. Music quality was subpar, and often took 10 seconds to load a song over my Verizon DSL connection.