We use one from time to time, it can be excellent, but has limitations. Basically it will let you see heat in line of sight - day or night. So whatever it sees can be seen with Binnys or a scope. It is excellent at seeing animals sulking in undergrowth or forest, but you will not be able to shoot them until they actually come into the open.
It is excellent for tracking, blood shows well as does an animal lying down, but you need to be able to physically see it, it overcomes natural cover and colouration.
It is a tool in the box, and for us it is best at showing what is not there, if we scan a field at night and there is no heat trace then there are no foxes, we dont need them looking into the light to see them. If we scan the hill before first light, we know if the deer are out and can get into a shooting position, just like someone who can get to the hill every day. To date we have not shot a single thing extra that we would not have before, but it has shown us where wildlife is moving, and this has surprised us, we have found both deer and foxes in areas we would not have expected them to be.
I am sure some will think it is cheating, in the same way that in the past rifles, scopes, night vision, glasses and lazer eye surgery and the like were seen as cheating. i dont have one, but if the price was right i am sure i would, it also is great for checking for dodgy wheel bearings, blown exhausts, sticking breaks, leaking pipes, electronic problems, poachers, lost pheasants and where your grouse are hiding!