I have Goodyear MT/R-Ks on mine currently (35x12.50R15). Before these I had BFG KM(1)s (same size).
Overall I've been happy with them. The tread is holding up much better than the BFG's tread did (severe chunking from rocks). I think the BFGs gave me about 1 MPG better mileage though, as it's down about that much over the last 1½ years and that's the only thing I can attribute it to, as nothing else has really changed.
The MT/Rs seem about 10 lbs heavier and have roughly an inch wider tread. Not sure, but maybe it's possible the kevlar belts have a higher rolling resistance also.
Offroad its pretty much a wash between the BFG and MTR, both tires grip pretty good, especially on rocks.
A buddy of mine has BFG KM2s. His tread is holding up better than mine did (running the same trails), maybe they fixed the chunking issue with the KM2. For some reason the KM2's appearance seems a little bland to me (sidewall in particular), I liked the looks of the KM(1) better.