Infrared temperature guns are a valuable tool to monitor and verify the correct temperature of the habitat, basking areas, thawed prey items, incubating eggs, etc. They are not a replacement for permanent thermometers/probes. What they are is an additional tool for temperature monitoring and double checking the accuracy of the thermometers. The market has grown considerably and prices have come down in the process. You can spend from a little more than $10 for a very basic, general purpose unit to over $100. As the price goes up, so do the quality, accuracy and features. Laser sighting is a common feature and is invaluable in knowing what area you are actually testing when you are not able to hold the gun close to the desired target. Another helpful feature is a more narrow distance to spot ratio. As the tip of the gun is moved further away from the surface, the wider the area becomes that the unit is recording. A distance to spot ratio of 1:1 means that at 12 inches the unit is checking a 12 inch wide area. A unit with a distance to spot ratio of 12:1 woud be measuring a 1 inch round area at the same distance. The 1:1 unit would be less accurate if there are items of varying temperature and you are not able to position the gun closely to the target.
In shopping around you might also see references to emissivity and accuracy. Emissivity of 0.95 seems common and will suit almost all of the users out there. The Accuracy gives a range of error in either direction. Most seem to fall within +/-2 degrees and that is fine for most of us as well.
The bottom line is that even the most basic units are beneficial and it really comes down features you actually need. A unit with no laser sighting and a 1:1 distance to spot ratio may be all the you need. Take a look at where you will be using it and what areas you will be checking. From there you can go shopping for what suits your budget and needs best. And please check with your favorite reptile product suppliers before you consider purchasing elsewhere. Many of them have their own custom units, with features and size tailored for the way most of us will be using them. It is important for us to support those that support our choice in pets.