Plastic kegs are starting be become available and soon may be the only way homebrewers keg things. At the moment, however, the keg of choice is the Corney. They're easy to work with, readily available, and reasonably priced. Sanke kegs are what is mostly used commercially but they cost more and are harder to clean. The reason the commercial guys use them is because they're tougher and are easier to clean IF you have the big industrial washing setups.
Right now the only plastic kegs that have made it big time are commercial, with the Sanke fittings, and intended for one time use. There is one I've seen that's targeted at homebrewers, but it's not much cheaper than a corney after you factor in the fittings and its just a rebranded version of those one time use kegs. They don't specify the expected lifetime. Personally, I'm sticking with my cornies.
