Is it better or worse than deviantart? I only ever stumble across tumblr when I'm looking for nice pics of my favourite characters in any given fandom...
But seriously, they should totally include a flock of crazy fan girls worshipping the pretty genocidal maniac in the next movie! You know, fan girls turned evil minions...
OMG it is a roller coaster riot compared to deviantart which is now old school. It's a huge scrapbook of fun. Just sign up and plug in anything you like into the search (works best with google chrome) (I say sign up because I've tried searching unsigned in and you don't get all the interesting things).
Very creative people. And just, PICS. Also pretty NSFW if you look for it, and sometimes if you don't.