I've been wondering about this, too and, as PGKEVET very thoroughly answers, it depends what you're looking to achieve.
I've been using a very low-cost quad-copter (£150) to record building progress on a house over the last year and here's what I've learnt about their use:
1. Make sure your camera is vibration stabilised - ones that aren't are pretty much unusable.
2. Get one with altitude hold - it's a lot easier to fly.
3. Heading-hold is also useful - people rave-on about "headless mode", but I found this to be a pain to use.
4. The wifi link on mine drops out at about 10m altitude, which is useless for FPV control - get one with better range if you want to fly off the video return or a headset.
5. Even a fairly light wind at ground-level can be really quite strong 20m or so up - my small quad-copter has been uncontrollable and blown out of range more than once - probably the bigger the better is the rule to use, there.
6. The cows and sheep I've flown over aren't bothered at all by my small quad-copter - something bigger or at lower altitude might worry them, though.
I've looked into IR filming of land briefly, to assess moisture levels, but it's quite technically demanding and the camera equipment is very expensive.
There's some interesting info here, though.
I would have thought a medium/high-end FPV equipped drone would be great for finding missing sheep up a mountain, though