I've got a collection of 19.2v Craftsman tools (drill, jigsaw, flourescent light, 7" circular, sander, spiral saw, and 4 port charger. I've got the big Lithium battery, small lithium battery and 3 NiCad's.
That being said I don't think I'd buy them again. Why? Sears seems to be abandoning the C3 line. You can't get any of the "different" tools - the spiral saw is nowhere to be found, they had a cool fan, there used to be a caulk gun, airless paint sprayer, staple gun, and a few other neat tools. The Craftsman C3 line is made by TTI and they also make Ryobi and all those tools are still available in Ryobi. I'd look into the Ryobi 18v line as it's a decent line and you get a lot for the $.
Performance wise the tools have been great. We're in the middle of a gut and remodel of our basement. I've been using the tools heavily and they have not disappointed. The circular will blow through a NiCad like crazy but with the big LiIon it's pretty serviceable for 2x4's and the occasional sheet of plywood. The drill has sunk tons of drywall screws and the sander has sanded a bit.
Another option for occasional use would be a 12V system. I fell in love with the compact size of the tools and have started a collection of the Milwaukee M12 system with a hammerdrill, impact driver, and oscillating tool. They have great power and decent battery life. It's something to think about.