I've only used one on bass... it's nice. It supposedly rearranges the way the signal's frequencies that come out of it in a more orderly fashion which in turn makes the amp sound "clearer" and yadda yadda. I do like what the pedal does in a bass guitar's signal chain, but in a nutshell, I find it's just a fancy shmancy midscoop.
Dues Ex Machina is right about the techy bit! although not sure I agree about the mid scoop bit :D
I bought one when a mate was selling it, he wanted his rig to sound a bit more muddy so got rid.
Basically I was told "it just makes it more awesome! try it out" I did and it's the one pedal that just stays on all the time. I love mine and wouldn't sell it
It just seems to add clarity to every note and chord, difficult to explain though and pretty much would advise looking to youtube to see what you think.