Spent over an hour adding content on Last.fm and am listening to it now. I really like it for active listening as just the included pictures / bio information is worth the time spent with it.
Not sure it's worth it for passive listening as I'm not sure I want my TV on all day when really I'm just listening to the music.
Last night the service was commercial free with just placeholder spots. I see that MS is running real commercials this morning.
Will it replace my Sirius? Nope. Like 1 vs. 100, this will be fun and interesting for a few hours and then on to the next thing.