Not too familiar with other brands, but years ago I got a deal on a Flir E4 that I couldn't pass up on even though I really have nothing to do with it.
One of the key things I liked about Flir is the MSX tech that puts an outline of the objects in the image (see post by RPH). I have looked at other imagers at the time, and it makes a HUGE difference on making out exactly what you're looking at or what part of the object you're looking at.
On other units/brands without it, a lot of times all you see is a blur of colors and it can be difficult to weed out the specific part you're looking at.
That being said, I have noticed some parallax error at times between the image colors and outlines, and I don't know if other brands have "caught up" with that tech. At the time I bought the E4, Flir units used the best/most accurate thermal imaging chip available.