I have used them in the past. They are basically just Type-K thermocouples. Mine have welded in bungs in the downpipes. They were difficult to correlate temperature to mixture, as was affected by load too. The temperature could be high or low depending what was going on. In the end I took them off as never saved a blow-up.
I started looking at detonation detection equipment, but not completed it yet.
Go-kart AFR systems might be worth a look too.
"Every day spent above ground is good. Every day spent on two wheels is better."