These thermometers are not very useful for brewing. The problem is that they read the temperature of a surface. Steam and vapor will interfere with readings as will reflective surfaces. Most of the IF thermometers aren't very precise. I bought one thinking it might be a good way to measure mash temps, which it is not. I have found a few uses for it though. I check the wort temp coming out of my CF chiller and in the fermenter as it fills. Works well for that purpose. I also use it to check my grain temperature before doughing in so I can figure out my strike water temperature. They are fast and convenient for some tasks, but not for all.
The accuracy is usually about +/- 2%. Not great, but close enough for some measurements.