I will ride down into the 40°'s without much discomfort....especially with the 'heat issues' common to the Spyder (actually, a godsend in northern NY this time of year!) But don't like to have cold hands!
I've used the Symtec grip heaters before and they seem to work really well and easy to install. Pull the grips off, clean up the handle bars removing the ribs on the throttle tube with a razor knife, stick the heater pads on, wire them up and that's all there is to it. There was right side switched power behind the right headlight (red wire) which made switch location easy and looks factory.
Tried them out yesterday and my hands were hot!! 'Course it was almost 70° out but....hey, the hands were warm!!