It's actually quite interesting to watch your listening habits and stuff for the week. I've realized lately how weird my musical tastes have become because of this.
People have also created pages that will take the info from your last.fm page, do some fancy math with it, and come up with stuff like an "open mind index"(which rates your open mindedness towards musical styles).
Overall I think the most useful feature on the site is the similar artist lists that they give you. The lists are generated by tags which are created by users(not some random idiot) so they tend to be really accurate(unlike most music sites). So if you feel like listening to something new you can just click onto one of the similar artists from a band you like and then go and youtube some of the popular songs from that artist(which are conveniently listed on the page for you) to see if your into them or not.
Last.fm has more features like ranking your similarity to other users, concert/event listings, etc. etc. etc. Too lazy to explain in detail about the rest so yeah just sign up if you haven't and try it out.
PS no this isn't an ad for last.fm I just think the site is cool, oh and my page is last.fm/holymnky