Overall, Acurite is the best digital thermometer you can get. But, it's true, from time to time you encounter a unit that either A) is bad from the start and must be replaced, or B) gets wet and starts reading crazy things!

As was mentioned, they also go haywire when the battery is low...which is how I judge replacement time. I have a PE1 temp gun as well, and whenever I can use it, it's usually within a few degrees of the acurite.
My biggest problem with the gun is that I can't figure out how to use it properly in my rack! To get the thing in close enough for a good reading, you have to open the tub almost all the way. In doing that, it takes the tub off the heat source, so the reading is defunct. I'm trying to figure out a wand of some sort to attach the gun to with a lever to raise to 'push' the button so you only have to open the tub an inch or so.

So, Acurite is good...but you have to be aware for the little things that can happen.