I ran a set of Dualstar heated grips on my Monster for 3 years. They worked remarkably well for $30. Install was easy and took maybe 2 hours. My 2000 Monster has an auxiliary loop built into the wiring harness specifically for extra goodies like heated grips. All in all, the Dualstars were money well spent.
However, I have a set of Oxford Hotgrips on my ST3 which I installed early this spring. The Oxford Hotgrips get quite a bit hotter than the Dualstar heated grips. In fact I've never run the Oxford grips at 100% heat for more than 10mins at a time. Also prefer the thicker-than-stock Oxford grips which fit my hands better.
I prefer the variable heat dial on the Oxford system to the junky Dualstar Hi-Low toggle which needs to be mounted into a fairing. Also - and this is for the newer bikes with fancier (read: scary to tamper with) wiring harnesses - it was nice to run the grips directly off the battery for ease of install. The Oxford grips are supposed to shut off automatically when they detect a low amperage from the battery. Never tried that, but it's a nice feature if you forget to shut them off.
However, in my opinion the best way to keep hands (and body) warm is a combination of hand guards to keep the wind off, warm gloves, a heated vest to keep the core warm so blood circulates to the extremities, and heated grips. Those things keep me warm on well rides into December here in Nova Scotia.
