Coolest thing? No idea, there's just so much you can print. You can do pretty much anything. Take a look here for inspiration:
http://www.thingiverse.com/explore/popular/page:1
How long does it take? hmmmm.
It varies alot on the settings you use - the % hollow, quality, layer thickness. But pretty much on normal settings, it'll take you ~45 minutes to print a phone case, and maybe an hour to print a low polygon squirtle? http://www.thingiverse.com/thing:319413