About a year ago I switched from glass carboys to using 1/2 bbl (15 gal) Sanke kegs for primary fermentation, and I absolutely LOVE IT. I brew ~10 gallon batches, and usually end up with about 11 gallons +/- in the keg when I pitch the yeast.
I have not tried the "pressurized" fermentation method, so I can't speak to that topic. I use the Sanke keg
fermentation kit from Brewer's Hardware, and it works great! After primary fermentation is complete, I hook up my CO2 hose to the off-gas port and use 2psi to "push" the beer from the keg to the secondary fermenter. No lifting the heavy keg, and no siphoning required. I have tried the same method using the cheap carboy caps stretched over the top of the keg with the same results.
At the start of my brew day, I place my Sanke fermenter (read: clean, empty Sanke keg with stem removed) on a burner with about a half gallon of water in the bottom, and boil for at least 15 minutes to sanitize with steam. After 15 minutes of boiling, I take the keg off the burner, dump any remaining water, and cover the opening with a sanitized piece of foil. By the time my wort is cooled and ready to drain into the fermenter, the keg is cool and ready to receive.
When fermentation is complete, I transfer the beer from the fermenter as described above, and then immediately clean and rinse the keg using my homemade keg cleaner. After the keg comes off the keg cleaner, I use a flashlight and a little mirror to confirm that there is no gunk hiding out inside the top of the keg. When the keg is clean, I slosh a little Starsan around inside, and then store with a stopper in the top. On brew day, the keg is clean, and should be sanitary, but I use the steam method just to be sure no nasties snuck in there during empty storage.
For me, this method was a major improvement over using my glass carboys. Easier to clean, easier to transfer, and no worries about breaking glass. Only thing better IMHO is a stainless conical, but I can't afford that bling!
-Eric
