I was among the early adopters of last.fm, few years back. But had shifted to Pandora due to its clean and uncluttered interface. Ever since Pandora started restricting non-US residents from accessing their collection, more and more users like me have shifted our allegiance to services like last.fm, anwyhere.fm, yahoo launchcast etc.
What sets last.fm apart from the competition is the massive community following. I had been using last.fm plugins for amarok and exaile to get my share of last.fm.
I recently tried the official last.fm linux client and it is surprisingly pretty good.
Best part of last.fm is the neighbours' playlist. Last.fm automatically picks people who match your taste of music. Give it a try, if you still havent.
I just wish more and more services start supporting the Linux Community. And for a start, it would be so great if we can have Silverlight and Adobe AIR running on linux. Popfly is amazing and I really need the Pownce desktop client.