I used 5 gallon cornies for about 40 batches, and at this point would recommend against it. ...
I've been using cornies for about 10 batches now, and haven't experienced a single one of these problems. But I do a few things differently to prevent the issues you've had.
1. I use a hop bag (for boiling and dry hopping), so hops never have a chance to clog the posts, dip tubes or relief valve.
2. I cut the liquid tube 1.25" from the end. Once fermentation has stopped and the yeast has settled out, I get very little trub or yeast being sucked into the liquid tube for transfer, essentially eliminating any chance for clogging. This generally only results in the loss of a couple of ounces or beer. I always use CO2 to transfer under pressure.
3. I use Fermcap to keep blow-off to a minimum. Its not perfect, and sometimes I still get some krausen in the discharge tube, but I've never had a clog of either the gas post or the relief valve. And I just hook the discharge tube (1/4" bevlex) straight to a gas QD, so its not like its a wide-open tube or anything.
4. I check for leaks after filling the keg by pressurizing it. If the lid or posts are leaking, I'd know and can lubricate them or adjust them as necessary.
Regarding cleaning, I just don't think its any more difficult than a carboy. Mark's keg washer works just as well on it once you've taken the posts off. And instead of having to put a cloth inside and madly swirl it around to get tough deposits off, with a keg you just reach in and scrub. Cleaning the posts and dip tubes takes all of 30 seconds with the proper tools.
Regarding temp monitoring, my controller has a steel probe which I attach to the side of the corny with long twist-ties and a bubble wrap pad. I'm not even sure the bubble wrap is necessary though - the thermal connection between the steel probe and steel keg should be a lot stronger than with the surrounding air. I sanity-check that its working properly with those stick-on thermometer strips.