My friend's got one, I drove it for a bit. The torque is other-worldly, but, the fit & finish was just---blah. Seriously. After gettin' back into my M3, I felt much happier.
If you're a horsepower freak then you shouldn't be in an M3 to begin with but I'd be willing to bet if you drove a GT500 for a week, then got back into your M3, you'd be grinning. Just my .02