Over the years, I've had two sets of H-D brand heated grips, and one set on Heat Demons.
My first set of H-D heated grips was a long time ago, on an '06 Softail. They failed after a couple of years. BUT-- I believe those grips have been completely re-designed since then.
I currently have the H-D heated grips, (had them installed on my 2017 FLTRU by dealer), and I'll go along with everyone else, I never have to set them above '2'. They're only a year old, but no problems yet.
I had Heat Demons, which are an aftermarket brand on my Street Glide. I installed them, (no big deal). The Heat Demons operate on a four position switch, rather than the dial vernier type switch that the H-D grips use. I thought they were fine, I had them for three or four years, always worked great. They come in black or chrome and you can get the Control switch for either the left or right side of your bars.