I bought mine because I know more about car engines then 2-strokes, but i've been riding for almost 10 years now on ski-doo 500's and 700's, and I wanted something new/different..
I think the engine is solid, and not going to cause any trouble at all, the turbo is smooth, and it handles very well for it's weight. I have only more than 200 miles on mine now, and I have to say it has it's problems (minor things that can be caught, or the dealer will fix, see the 4-stroke forums..). They need attention, and can't be just ridden without checking over between rides. When you ride it, it just makes you happy, opening that throttle, it works really well, but you can't help but realize what's going on under the hood isn't simple, with the turbo kicking up to over 100,000 rpm, the engine itself over 7000rpm (how many car engines are happy all day at 6-7000+rpm being force-fed 20PSI of boost or more???) etc.. lots COULD go wrong, so it must be in your mind when you are riding it. The turbo has to be cooled, and the oil must lubricate it, if either one goes, you'll end up with no more turbo, and I don't want to know what the 660 is like running only 6:1 comp without the turbo to get you home (probably like 30hp). This is all just to let you understand the bad side of what could go bad, don't forget many 2-stroke sleds could have worse failures just by a temp change and wrong jetting, otherwise it's a great machine, very unique, and the design of the engine has been improved and proven in racing and road all over the world since 1989.
bottom line: if you like to really beat your sled, and race more often than not, buy a great performance 2-stroke while they are still around. If you mostly cruise at high speed, and put on big miles, plus you want something that starts easy, is very different, and still goes plenty fast when needed, then consider a 4-stroke.
mine has only had a broken ski handle, and the caincase oil drops slowly, that's all.

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