Thermal works basically on the principle of differential temperature, you are pretty much seeing heat.
IF you are trying to see a critter with a skin/fur temperature of 95 degrees in an environment thats 95 degrees ambient you're not going to see much if anything.
From my own use and observation on a couple of these systems they work very well at night or on cold days when trying to spot critters or any heat source. They do not work real well in dense fog or on hot days with objects in direct sun light (radiantly heated). They do not work well viewing through glass (from inside a vehicle or house looking out).
They are expensive, a fairly good model will cost $14,000 with a high end model costing $35,000 or more.
Most models don't offer very high magnification and if they do you're going to suffer from some pixelation/jaggies (look these words up on the web if they're new to you).
Raytheon makes one model I used for a night shoot on deer, it was an early variant of the heavy AN/PAS-13. This is a nice unit, very clear but heavy at about 5.5 lbs.
Personally, I would not get one for trying to judge critters at long range but in the correct conditions they work great for spotting any heat source out to very considerable distances. A great use for one would be to scan a meadow or field for any live object providing it's a cold early morning just before sunrise... It'd also work great for deep woods scanning for critters.
As with most monocular equipment you'll have zero depth perception which will made range estimation difficult (at night).
They're a real neat toy so long as someone with deep pockets buys it and lets you play with it.