Actually there are two main ways I've been using in this service. Sometimes it's just fun to listen these really popular stuff and for that I just select popular from left column and hit play all. OK, there are lot of stuff I don't really like, but I can skip those easily.
Second case is, I hit Radio button low right and let it start playing different songs for me. Then I'll teach it a bit by liking and disliking some songs and soon I start to get proper music for the current mood. It's just so awesome.
If I wanna listen some specific artists, then I use other services. For me Grooveshark just works perfectly for discovering and listening some good music. I've used it so much lately, I thought it would deserve blog entry.
Give it a try, if you are not familiar with that one already.