I've done it before, but wouldn't do it again. For one thing, you'll get maybe 4-4.5 gallons out of a corny keg (5 gallon keg). For another, you need to modify the keg in order to ferment in it. At the very least, you pull the gas post, slip a short piece of tubing over the fitting, to secure the airlock to. Or you get one of the fermenting lids, or make one yourself. Personally, more trouble than it's worth.
I DO ferment in adapted sanke kegs though. I use tall 1/4 bbl kegs for my normal size batches (of beer). I have an adapted 50L keg for larger batches too. I also have several 1/6 bbl kegs that I use for mead and such (smaller volumes). I've adapted these to include a gas post on the top of them. I then use one of my sanke fermenter caps (my own design) so that I can ferment in them, have a thermowell in the middle, and then use CO2 to push the finished beer out of the keg. I have dip tubes cut to leave a certain amount behind in the kegs, so transfers are pretty easy.
If you can't score some sanke kegs for fermenting, then I would advise continuing to use what you are now.