Well, I did my brew today, and of course I forgot to test the IR thermometer until part way through the boil. There didn't seem to be much point in measuring the temperature of boiling wort, but I did it anyway. The temperature (according to the IR thermometer varied from 165 to 185 degrees F. Not a very promising start. (I would expect in incorrect temperature because the device was not calibrated for this use, but variations of 20 degrees seems to be somewhat excessive, although it was very windy where I was boiling, and this may have had some effect)
I then used it to take the temperature of the wort while chilling with an immersion wort chiller.
While chilling, the device returned consistent temperatures. At the start of the process, the temperature was within two degrees of the thermometer built into the brew pot (which is accurate to within one degree F at both 32 and 212 degrees).
As the chill progressed, the difference between the two thermometers increased until the brew pot showed 76 degrees, while the IR device showed 120! I believe the reason for this discrepancy was that although the wort had cooled to 76, the brewpot above the wort level was very much warmer, and the thermometer was measuring the pot temperature above the wort level, rather than the wort temperature, although it was pointing at the wort.
If anybody wants more test results, I shall be brewing again in a couple of weeks, and I shall try to measure the mash water and mash temps, but unless somebody specifically asks for this data, I think I shall agree with the previous posters, and conclude that it isn't worth the effort.
-a.