I've been riding for the past four years or so. The main reason I started riding was the fact that I was spending way too much time inside during the winter complaining about the cold and snow. Snowmobiling for me has been a great way to get me out and about in the winter and to see areas that may or may not be easily accessible on foot or by other vehicles. It has a certain appeal like geo-caching . . . it's not for everyone.
Folks that enjoy snowmobiling often tend to be the type that also enjoy riding ATVs or motorcycles. In fact, snowmobiling is similar to riding a motorcycle on snow . . . not necessarily a good thing since as Dave mentioned a lot of folks go pretty fast. As for me, I tend to be a slower rider than many . . . ATTROLL can attest to this since I went on a ride with him once.
One common belief though is that snowmobiling is cold . . . if a snowmobiler is cold it's only because they've got the wrong gear. I've often gone out and been cold on only a very few occasions -- usually because of a poor choice in clothing. When you bundle up with thermal underwear, polar fleece, outerjacket/pants, helmet, balaclava, good gloves, etc. and turn on the heated hand grips you're often toasty warm.
Snowmobiles can be noisy and smelly . . . particularly the sleds that are using older technology. Newer sleds however are designed to burn cleaner and the four stroke models (most new Yamahas for example) are not all that smelly and are really quiet. However, if you want to get out and about with zero smell and little noise snow-shoeing and cross-country skiing are great outdoor activities as well . . . they also offer the added bonus of giving you a better work-out than riding a sled (which may require some finess, but nothing like snow shoeing and X-country skiing.)
Snowmobiling can be expensive. I originally bought a used sled for $1800 and then bought a new sled for around $6,000 a few years later (not the biggest and best of course.) Getting outfitted in gear can be cheap or expensive -- I probably have around $400 in gear . . . mostly clothing that I use for other activities as well. Some folks go crazy and get the brand name stuff which can add up to a alot. I also bought a trailer for $1,500 which I use to haul both my sled, ATV and "stuff." And then there is the miscellaneous expenses like gas, oil, registration, eating out, etc.
"Courage is not the absence of fear, but the realization that there is something more important than fear."
"Death is only one of many ways to die."