I recently bought a QUICK 191AD soldering iron thermometer, here's a 3-minute review : )
This instrument costs about £100 in the UK (a couple of resellers in the UK stock it). It comes with a pack of ten sensors, there is a spring peg to hold it taut. It's a type-K thermocouple basically.
I cannot say it is the most useful tool in the world, I'm unlikely to use it often just to check soldering iron tip temperature. I may get a bit more use out of the tool simply because it has a connector for a normal external thermocouple probe too. One is supplied with the tool.
Functionality is pretty basic; all it does is display the temperature in degrees C (1 degree C granularity) between -13 and 973 degrees C, there is a Fahrenheit button, plus a useful Max-Hold capability, and the power-off time is configurable. The internals show that the design is quite ancient:
There's lots of spray coating on the board, and I couldn't read off any IC part numbers, unfortunately.
The instrument comes with calibration, and it is accurate (I tested with a thermocouple simulator/calibrator tool) across the entire range. My soldering iron was measured by the instrument to within 5 degrees C of the set temperature.
To summarize, the instrument is quite basic, but since it comes with calibration, is accurate, and can be used with an external thermocouple, I think that goes some way toward making the tool more useful.
Thanks for reading!