I’ve been playing with last.fm. It has some nice features. It has some bad ones.
For instance, when you start using their software, it bootstraps your listening history from iTunes. Trouble is, if you use iTunes on more than one computer, it bootstraps only the first one you use it on. I didn’t know that and started with my laptop where I have much less of my music collection stored. Now there’s no way to get my real listening history off my desktop computer, unless I cancel my account and start all over.
They could learn a lot from LibraryThing. As implemented, they have no way to merge multiple spellings for the same artist (so for instance, they have “Interrobang Cartel” and “Interröbang Cartel”). Nor can they disambiguate artists with the same name (so for instance, clicking on “Calliope” gets you to a “profile” that starts off telling you there are many bands named Calliope). They say they’re working on solutions; these are problems that should have been addressed in the design, not patched up afterward.
And of course, as is almost always the case with digital music services, they really haven’t a clue how to handle classical.