I've used kegs to ferment in, both corny and Sanke... I've also aged in both kinds of kegs. I don't "secondary" my brews, giving the full time span on the yeast before aging/bottling. I'm actually going to have a batch going into a 1/6 Sanke today (starting the batch shortly) that will sit there for about a month before getting bottled.
Personally, I really like to ferment in kegs since they're SS, light (and air) tight and are easy to find places for. Plus, they have handles built in, making them very easy to move. I'm also planning on getting some 1/4 Sanke kegs for bigger batches.