Also know as the HET. I got to drive a few when I worked for Oshkosh Truck in WI. (Still love HEMMT's the best!) The experience was like driving a building from the second story window. When you turn, you have the feeling that you're moving sideways more than turning...
And you look down on normal semi trucks like they look down on cars, which is interesting. I'm 6' tall, and if I hunched just a tiny bit, the door would swing open OVER my head. I don't recall top speed for the HET, but I think pmoyle is close at 45 or 50mph.
And they have not one, but TWO 50k lb winches on the back.
The trailer that goes with these trucks is amazing. Has 40 tires paired up two to a caster IIRC, and all the casters swivel as the truck makes turns, and are suspend to follow terrain very smoothly. And the trailer is 14' wide. LOTS of hydraulics under there!!
Actually, the better deal is the M9XX series trucks... (923/933/925 etc) They're capable of close to 65mph, and fairly easy to drive, and also VERY cheap. M1078 Stuart and Stevenson trucks are starting to come on the market at reasonable prices too, but they top out around 55mph, and use a bit more fuel I believe. But they would make a great chassis for a large overlanding rig!
And I'm still holding out for a HEMMT, preferably with the rear mounted crane.