I found the concept for this creature intriguing. I'd be interested to learn more about its back-story -- why is it forced to do what it does? Perhaps an adventure could be designed around unraveling this mystery and setting the creature free.
The only thing I didn't care for was that the creature takes on the stats of the PC it fights. As a GM, I don't know my players' abilities, spells, feats, powers, etc. nearly as well as they do. I'd be overwhelmed if I suddenly had to control a monster that had a PC's character sheet. If I were to use this creature, I would work to simplify the PC, boiling his/her abilities down into a more manageable package. That could be tough to balance, but I still believe it would be the better alternative.
Other than this fairly minor complaint, this is a worthwhile creature to download and take a look at.