Personally, I use them because when I was in theory, my hands were always sweaty for some reason and my fingers kept slipping. I use the thick rubber ones, and I think it makes a big difference. Also, when I bought a newer machine, I had to use it for a few days without them, and the vowel keys made the sides of my thumbs hurt a little, but that's just my opinion.
I've been using them for years, and I love them. I like the thick rubber pads, personally.
The pros are comfort and no slipping, softer touch. The only con is some of them are a bit too big and have to be cut back so they don't bring down surrounding keys at the same time. If you get them, just be sure they're not touching other keys.
Once you use them, you'll never want to go back. They're worth it.