I can't vote because mine have only been "working" for 7 months, and of that time, I've only needed them during the past two months.
I would comment however that I do not like them as well as factory heated grips I've had with another marque.
My problem is the continuously variable power switch. It is hard to change the setting while riding, and Yamaha recommends against this. Probably more importantly, it is desirable to be able to change the setting while riding because the grips can become overwhelmingly hot at the higher settings in less than arctic air. I need the grips a few days ago (I tend to go for some heat when riding temps are below 50), and although I had picked an intermediate setting, they turned my Helm gloves from dark brown to a grayish tone. I think the gloves' color will come back with some neatsfoot oil, but fine tuning the potentiometer seems to require some skill. Part of this may be because not only is there a potentiometer for the rider to control, but the bike is also modulating the power to the heated grips, dropping to something close to 0 when you come down to idle speeds. I think its overly complicated, but with that said, mine do provide heat.
On another bike I've got a simple three position switch right by my right thumb. Off - Low - High. Low seems about perfect, and on low I find myself flicking it off, or on, during the ride. I am beginning to think that an easily accessible on-off switch is better than a fancy controller..... same with my Gerbing's heated jacket liner and it's fancy controller.