I finally got around to installing and trying out the Oxford Hothands OF694 that I purchased almost a year ago. You can see them here: http://www.oxprod.com/
These wrap around your existing grips and secure in place with velcro. The wiring is simple - to install properly you need to remove the seat and tank. I mounted the switch on the right handle bar which, unfortunately, has only two positions: off and on. However, once turned on my hands began to feel the heat within five seconds (really), I was not wearing gloves at the time since the temperature was about 25 celcius. They should be good enough to take the edge off when the temps get really low.
I'll update this topic once I have had a chance to use them in colder weather, with gloves on.