I've bought a new Skidoo 800 almost every year since 02 (02, 04, 05, 07, 08, 09) and I put several thousand miles on each year. Yet to have a single engine problem. I guess I am just lucky to not have had engine issues. About half of my friends also ride skidoo 800s and none of them have had issues. I do have to admit that I did own an 05 skidoo 1000 and it was a piece of ####. It was just too damn heavy. No engine problems, it was just heavy and cumbersome. But as far as the 800s go I have been extremely pleased. The new XPs are pretty awesome sleds and I have yet to see a stock sled that can highmark me (that includes dragon 800s, M8s, and M1000s).
You defintely have to consider the snow you will be riding though. If you have very deep light fluffy pow and not as much riding experience I would reccomend the M7 or the dragon 700. They are easy to maneuver and easier than an 800 to dig out. If you are an experienced rider an 800 is definitely the best sled in my opinion.