I like Pandora much more than Last.fm, actually. Like you said: it's immediate. It's also simple, and it provides you with a lot of pretty good stuff. Last.fm, while it's a good concept, does much more than I want it to. It also gives me the feeling of being watched: keeping a record of everything I listen to, much more putting it up for everyone to see? I'm not out to impress everyone (at least everyone on the net) with what I'm listening to, nor do I want to be judged if I occasionally want to listen to something "unimpressive." So far, I haven't noticed better results with Last.fm radio, either.