The Haaglunds seem to have a following, but I think they're mediocre at best in deep snow. I've posted this link before, but if you're even remotely considering one of these machines, the video will not make you want to rush out and buy one. If anything, it's great advertising...for every other brand of snowcat:
Bill Feeley, the man behind the "Extreme Haaglunds" name and business, passed away earlier this year. I have no idea if the business is still operating, or if it will continue to do so. I would think there are some proprietary parts involved in the conversions and if you need one, or more, in the future you may very well be out of luck. Then you're faced with trying to have your broken part reverse engineered and specially fabricated (read: $$$$).