It's not like this hasn't been beat to death already but...
The Pro 800 don't have reliability issues. Not really.
Have some burned down? Sure, but beware of what you read on the interwebz since bad news tends to collect in web forums. And how many of those "My buddy knows a guy who's Pro seized" stories are telling the whole story?
I'm 48 years old so you could say I'm a "mature" rider (don't say that to my wife) and I've got over 3,400 miles on a '12 Pro 800. I ride as hard as my body and ability allow (boondocking, WO climbs, easy jumps)
My sled was a bit overdue a top end at 2,200 miles when it went in for a LR conversion. Good thing too because my primary gear was 200 or so miles away from complete failure. I have not had a single hiccup, burble, or stumble before or after the conversion.
The Pro is a genuine light-weight high performance sled and like any finely tuned piece of technology with tight tolerances it requires careful upkeep for it to run properly and last.
That said, it's still a two stroke and will not run forever.
To greatly increase your chances of trouble free riding
-Turn up the oiler to a 40:1 fuel/oil ratio about 3 turns from stock (thread on how to do it is here on SW)
-Run ethanol free gas is you can, but always run the best gas available
-ALWAYS warm it up to temp before riding
There's a few other things but these three are key. With a turned up oiler, good gas a religiously warmed up sled will serve you well.
If you're buying used I suggest basing your decision on the seller's garage. If it's neat and tidy and it looks like the guy takes care of his stuff then he probably does.