To your main question - how reliable are they?
Unfortunately no-one here knows. They only know from their own experience and don't have any kind of statistical analysis to back it up. The manufacturers know and possibly some retail and rental outlets, but that information is not made widely available. "My camera broke" or "My camera has been great" even repeated 10x is nothing more than anecdotal, and is basically pointless.
The rational thing to do when you have a "lemon" is to get it exchanged or repaired. Switching systems is exactly the same thing as just rolling the dice again on a new camera with a new manufacturer. You are making an emotional not a rational decision.
Weather sealing is s complex thing. No camera works under extreme conditions e.g. drop it in a bucket of sea water for half an hour they all break. No camera stops working if one drop of water hits the front of the lens. They all break somewhere in between, and LUCK plays a big part. All that the manufacturers can do is add sealing and play the odds. So when it starts raining in the jungle and you are using a D80 there may be a 75% chance that it will stop working if you do not protect it over a 30-minute period, with a "weather sealed" K200D that chance may go down to 20%, with a Nikon D3 down to 2%. (Those percentages just completely made up.) But if you keep using it in the rain without protecting it, it will stop working sooner rather than later. For working pros (like photojournalists) who operate in extreme conditions they make sure they have multiple copies of the top-end Canon and Nikon cameras and the backup of Canon/Nikon professional services contracts to get them replacements and repairs quickly.
One of the biggest marketing scams in recent photographic history is Pentax managing to convince everyone that there are loads of old and new Pentax lenses available to use with their cameras:
1. A lot of the old lenses aren't very good by modern standards anyway.
2. Pentax have made 80m lenses in their history. Canon have made >100m AF lenses since 1987. Nikon similarly.
3. When you go to a 2nd hand camera store in any town or city - what lenses do you see in the window? Mostly Nikon and Canon, that's what. Used Pentax lenses are few and far between.
4. The actual range of new lenses available, though adequate, is certainly not in the same league as Nikon or Canon. But they do have a few gems. For most people who just want a couple of decent zooms this is not a real issue.
Having said all of that, I like Pentax, and the K20D in particular is a very nice camera and incredible value-for-money. IMO probably the best value DSLR currently in production.
I also sympathise with wanting something a little bit different. And most importantly a camera that you like using means you shoot more; it's quite sad when people say things like "It's just a tool." A nice camera should be more than that, it should be an object of fun and affection.