(Nearly) Everything said above is 100% true. You need to take all of that into consideration while making your decision. Especially the comments on your Dealer Support.
That being said, I will give you my personal opinion of each of the 4 makes of Mtn sleds, including Yamaha, because I've ridden all 4 and I own 2.
Let's start with Yamaha (2007 Mtn Apex minus 100#, no Turbo).
Pluses: High Quality, Solid Engineering, 4-Stroke: Tourque, No Smoke or oil to add/mix, Low Maintainance, Long Lasting, Easy on Belts. Semi-Rider-Forward Style Design (this can be a plus or a minus depending on your preference), Easy to roll over in deep snow.
Minuses: Heavy! Can be Lightened-up but only to a point and it takes a TON of $ to do so (don't ask me how I know this). The extra weight can wear you out on a big powder day, not the best track/traction, hard to side-hill.
Ski-Doo (2011 Etec 800 Summit X 163")
PLuses: Light, Less Smoke than any other two stroke, VERY Easy to Roll over in deep snow, Easy to side hill, Great Power, Great Traction, Rider-Forward Design.
Minuses: Unproven motor, Eats Belts? (not so far for me), Computer Break-in period (about 300 miles, won't give you full power until after).
Polaris (2011 Pro RMK Assault 154" Powder Track)
Pluses: VERY light, Easy to roll in powder, Good traction, Good power, Semi-Rider-forward Design.
Minuses: Motor History?.
Artic Cat (2009 M8)
Pluses: Great Motor, Reliable, Tough, Easy to roll in the powder, Good Traction, Traditional Rider Position (again, can be a Plus or a Minus), Fairly Light, Easy to side-hill.
Minuses: Due for a redesign (buy a new M8 this year and the 2012 is all new, now you are stuck with an "old" sled), not the lightest on the mountain.
All four are GREAT sleds. You just have to weigh all the pluses and minus and decide what's most important to you. Don't forget Dealer Support!!