Buy one, ride it, enjoy it, keep up on maintenance and odds are you will probably have relatively few problems especially with 2013-15. If you are nervous buy one still under warranty and then buy an extended warranty. Are there people having problems and reporting them here on snowest? Yes, there is with any brand, people some here looking for help or other people's experiences with similar problems. I recently picked up a used 2015 and so far I love how it runs and handles... if you are going to buy you won't do better on price than right now.
I was wondering how the pro rmk/ rmk 800 reliability is?
IMHO: Better than prior years, depends on miles / hours and how previous owner rode and maintained it.
How does it feel to ride?
Feels light, "flickable" - LOL - it depends on what you are used to, what sled are you riding now? Mine has clickers, definitely like the adjustability
Anyone have any good/bad experience with these sleds?
The forums are full of the good and bad, especially the bad since people are looking for help or wanting to vent when they have a problem
How do they perform in deep pow?
Again forums are full of info on this, especially when the ProRide chassis first came out in 2011, Google search of the snowest forums is your friend for better searches than the Snowest forum search engine. Haven't had mine in deep snow yet, but my 163 Dragon was awesome and the pro is lighter and supposed to get up on snow better
And finally when for you guys change pistons cylinders..
I personally believe pistons are way cheaper than dropping a skirt and blowing the crankcases, I would replace pistons when compression starts to drop or about 1500 mountain miles, would probably still use a "fix kit" even on a 2013 and newer. Cylinders just need a touch up in general so the new rings seat, and good to go for another 1500 (mountain miles)...
P.S. Your status shows you are sad because all the snow is gone... we are still riding here. At least this week.