Use the search, foo'!
Like Oompa, I've been using this since the days of audioscrobbler; there have been some odd bugs along the way, but by and large it works great. I always used the iTunes plugin until they released the standalone app that automatically scrobbles your iTunes plays; that seems to work fine too. And tagging your music isn't an issue if you've bought it all
Now that the iPhone SDK's been released, I cannot wait for a Last.fm iPhone app that will run on a device that isn't Jailbroken. iPod play counts are scrobbled when you sync, but since the music on the iPhone has to be managed manually, the Last.fm app won't pick up on it, which sucks. But! Hopefully an iPhone version will be out not too long after June
