I ran one on my 6.0L Cheby 2500hd and loved it. Tons of power, great drivability, and milage wasn't bad when I drove nice. I installed it myself with some help from friends, took about 10 hours but I was really anal about it.
Just recently helped another friend install one on his 6.0L Chevy 2500 and he loves it also.
Talked with a bunch of guys running them on their 8.1L and they have some sick power numbers. Make sure to get the intercooler, and if you don't have the Ally trans....save some money to get some work done to beef up the 4L85E tranny and add a larger tranny cooler. I hurt my 4L85E with just the 6.0L and Whipple and you'll have a ton more HP and torque.
PM me if you have any questions....
Good luck,
Chris