On trails in northern Ontario that use logging roads and visibility is excellent for a mile or so ahead 100+ can be fun and relatively safe, for a while. Please slow down when you cannot see beyond your stopping distance as others use them roads as well. A tight twisty trail at a fast but controlled pace is fun too. Out of control, ripping down trails is definitely an adrenaline rush, but the risk of death to self and others is ridiculous and irresponsible.
However, the most fun of all is, off trail, in the deep, at much less than 50 mph. There just is not anything else like it, and if you haven't tried it long enough to develop a little skill at it, you are missing a lot. The feeling of blasting through powder and around trees, up hills, along the side of them, and back down them is unreal. In addition, you can wreck your sled without seriously hurting yourself. Try that at high speed. Even if you are lucky, it is still going to hurt. I have the aches and pains from years gone by to prove it.