I have Tuffys in my mountain bike, and they have worked well. I've actually got an extra pair, and may put them in.

My bike came with slime tubes installed. They won't stop a flat, but I found that they would seal the hole, and I could pump the tire back up. But, I started having trouble with the valve stems leaking. So, my LBS put in some heavy tubes and a different sealer (I don't know which one). I had a few flats, but I was able to pump them back up. But, eventually one of the tubes wouldn't seal.