I tried to use it, once, but didn't get far in the "mining" process. And I never bought any, because it seems to defeat the purpose of using an independent currency to just throw dollars (or whatever) at a banker.
From what I can tell, it's a nice idea and I really like it in theory, but it's...a little naive. From the economics standpoint, there's a maximum number of "coins," which means the system will deflate, faster if it catches on. Either way, though, deflation historically ends very, very badly for the people using the currency. Prices stay high so the vendor can pay bills, but the value of the currency rises, making transactions more difficult.
I haven't heard of a problem with it, though, other than the lack of acceptance Watson mentions. And the places that do accept it are often...a bit suspect. So there's a bit of guilt by association involved.