It works great. I've done 13 gallons in a sanke with no problems. (use a blowoff just in case)
Only problem I've had is the temp-control. 13 gallons of wort has a huge thermal mass, and cooling is a bit uneven in my temp-controlled freezer. I let mine sit for a few hours before pitching, and shake/swirl it a bit during that time to even out the temp.
The thermowell fermenter cap is a great investment, buy one (or two) and don't look back.
I think it's easier to clean 1 sanke than 2 carboys, I use a sump-pump cleaning machine, and a homemade spinning carboy cleaner.