I agree with RonH regarding using a Gerbing or Warm and Safe controller. They are small and work great. Note that these are not rheostat controllers, which waster juice. They are pulse width modulators. When you have them at 100%, full power flows full time. As you dial it down, the controller pulses between 100% power and 0% power in ever decreasing time pulses as you dial towards 0. Very efficient. And those controllers are very small and can be mounted out of the way. That Yamaha controller is HUGE and takes up way more room than needed.
I have had bikes with heated grips and they just don't cut it in cold weather. They are nice because they are always there. You don't have to remember to bring them with like gloves. But as others have said, they just can't keep your hand warm like a heated glove when the weather turns cold. I have the Firstgear carbon glove (firstgear rebrands the Warm and Safe line of heated hear) and have been very happy with them. Last winter, I road the 30 minute commute to work daily down to about 15 degrees F. before the snow and ice storms stopped me. I was totally comfortable. That was without hand guards to block the wind. Heated grips can't compare, but they would make a nice supplement...only if the pricepoint is reasonable. If you don't have heated gloves, I recommend you spend the money on them and a heated liner before heated grips. A warm core will let your body continue to keep your limbs warm. A cold core cuts circulation to your limbs since your core and brain are you bodies priority. Note that if you get both heated gloves and a liner, you'll want a dual-controller so you can control them separately. I often run the gloves on 100% but usually only run the liner at 40% and occasionally up to 60%.
