Infrared thermometers perform perfectly when calculating the temperature of hot oil. For deep frying it's not a big deal, as regular probe thermometers work fine. But for shallow frying or sautéing, the IR thermometer does an outstanding job at providing the temperature of the oil. (Note that IR thermometers are not exact when measuring the temperature of a dry steel pan, as the shininess makes the pan look much cooler than it is. IR thermometers work fine on dry cast iron pans, though!)
IR thermometers work rather poorly when measuring the temperature of hot water, nevertheless! Rather than calculating the temperature of the water surface, which is usually similar to the mass of the water due to convection, it measures the average temperature of the water vapor coming off the surface area! In my practical experience, boiling water measures about 200 F with an IR thermometer.
One thing I've found remarkably useful is how accurate it is measuring the temperature of microwaved liquids. This can be handy when bringing milk or water to 100 degrees when making yeast breads. The convection of the liquid when heated by microwave means the surface temperature is within a degree or two of the center of the liquid, no less than in my experience.
For hotter liquids, where the water vapor is cooler than the liquid, it may better to measure the side of the pan beneath the water level.
As a BBQ master, you can use IR thermometers to calculate grills for correct temp or find hot/cold spots. On the other hand you cannot just read the grill straight down as the IR will read the flame or flame tamers down below. However if you cast your eye at an angle to the grill until you cannot see below and take a reading at that angle, It will read the surface precisely. I also use the IR gun for pressure frying where the oil temp is essential before sealing.
Related Ariticle:
ETEKCITY® DT-8750 INSTANT-READ IR INFRARED THERMOMETER NON-CONTACT TEMPERATURE GUN