Our patrol units routinely use handheld FLIRs as well as mobile truck and helicopter mounted ones. They are worth their weight in gold and in coal. I say that because like an earlier poster alluded to, they are only as good as the operator.
In the right hands, you can unleash war upon anyone attempting to flee. But in the wrong hands you'll be chasing ghosts or constantly hearing "Nah, I don't see anything..."
HONOR FIRST - WE DO BAD THINGS TO BAD PEOPLE
"Certainly there is no hunting like the hunting of man and those who have hunted men long enough and liked it, never really care for hunting anything else thereafter."
- Ernest Hemmingway