I have 1000 miles on mine and really like it. Bought at the end of the 06 season. I knew the motor was a one year wonder but if all goes bad, I could always change the top end to a 900.
After 150 miles, I replaced the intake with a Timbersled intake kit. Added an SLP pipe and silencer (with reprogram) Powder Pros and Holz Spindles. The intake and pipe took 27 pounds off the front. Skis and spindles made the stearing less work. I knew the sled would be heavy in the front and steering had been an issue for many. I did my homework and find that this set up works really well. Under 500 pounds, ton's of low end, easy to ride. For preventative measures, I had the clutches redone (see the clutching threads from Indy Dan) and added a vent (4" Flo Rite) in front of the clutch to keep cool air under the hood.
Ultimately, if you do most of the items above, you will allow the sled to run to it's potential without loosing reliability. Yes, it's a bummer that the out of the box config holds in heat, might be hard on belts and is heavy on the front. I got into it expecting all this. However, the price was too good to pass up. After most of the above, I have a sled that is easier to carve turns with and weigh close to the same as the 06 Summit 800 I considered at the same time. And I have close to the same $$ into it. I test rode both sleds, same day, same place, same variety of riding. The RMK was the better of the two to begin with. IMO - Better handling in the snow, better riding suspension when you have to ride home over the washboards down the mountain. The changes make it even better.
Ran my sled with a 07 Dragon 155 - if I had the extra 1/2" of lug (a 2 1/2" snow check track) I could have gone anywhere the 07 did. And gotten there at the same time. I rode the 07 too. The 07 felt like a typical domestic Polaris motor (I've had many Po 6's, 7's, 8's in the past and liked them). The 07 motor feels like it belongs in a crotch rocket, my 06 feels like a diesel.
Great sled if the price is right !