I owned a Timbersled Mountainhorse 120" and road it for 3 seasons. Had the kit on a KTM 525. They are great fun. To correct one comment, they do not have anywhere near as much hp as a snowmobile. Most dirt bikes make 50 or 60 hp. A 600cc sled makes 120 and an 800cc 165hp. Take any of them to 10,000 feet and lose 30% of that (non turbo/supercharged engines lose 3% hp for every 1,000 ft of elevation. Hp junkies tend to not like snowbikes, but i sure liked it. They are only about 100 pounds lighter than a mountain sled so still pretty heavy for the meager horsepower. Fun to wind through the trees in the flats, but even more so on a steep side hill traversing through the trees.
They are very easy to sidehill through the trees compared to a sled. It's kind of cheating as a guy on his second snowbike ride can go places a 10 year expert mountain sledder can go. With so little hp, you can not go straight up steep hills, but you can traverse up them. Also, not the funnest in superdeep snow 40+" since they don't have the hp. Deep days are snowmobile days. Very fun, though. I am back on a sled for the time being. Waiting for the next big snowbike advancement before I go back to one again.