Is Last.fm really as good as it sounds?

I’m really into music and am considering using Last.fm as a tool to find more great music. I’m contemplating joining the site and am wondering if any Flutherites have used it, and how good or bad their experience on it was. Is it worth joining? Are there hidden fees? Any viruses people have traced back to Last.fm? You could rate it on a one to ten scale, or just describe how awesome or lame it is.