I've used one of these for a long time and love it - simple and cheap. The tooth scores/cuts the sheath so it rips off but I've never had it damage any of the conductor's insulation. It works on 14/12-3 also, although it's a little fussier. With a little practice you can run the ripper down twisted NM and it scores enough so the sheath just splits off. The hole in the handle means you can run it up the cable if you're ripping off a lot of sheath.
http://www.ehardwicks.com/product_line/pros/KLN-74018.htm
