A number of years ago, I managed to score a pair of 10 gallon cornies that have become my primary fermenters. I have an extra pair of gas posts, one for each keg.
The posts have no poppets; they're just empty bodies. Each post has length of vinyl hose stuck on it as a blow off tube. I fastened wire around the necks of a pair of water bottles that I hook onto the kegs as the blow off tube reservoirs.
I typically brew volumes to put about 8 gallons of wort into the kegs to ferment. I've never has a problem with blow outs of crazy over blows.
Cleaning, as long as you don't let crud dry on, is not a problem. Warm PBW solution is our friend.
I did finally get a bit tired of scrubbing on the krausen ring, though. So, without knowing about "Mark's Keg Washer", I built one of my own. Even with a pump and other parts, I spend well under $40. It does a great job and I don't have to scrub anymore.
The old carboys are used for cider fermenting. Cider doesn't get light struck, so glass is not a problem.
In short, in my opinion, corny kegs as fermenters are far superior to glass. Any of the negatives presented in this thread are easily overcome.