Illegal, however, in the United States if there is any commercial use of the recordings. The FAA has recently made this very clear via stop and desist letters.
From a hobbyist point of view, the real-time video and other sensor data obtained from remote controlled FPV models (FPV = First Person View) is most impressive. Payload is several kilograms.
He hires out to Bay Area cities all the time. They don’t seem to think its Illegal.
I do topographic surveys by aerial photogrammatry; so do scores of others, daily. It’s very commercial, and advertised in the yellow pages.
Where do you think Google gets their data?
Is Google Map illegal? (LOL!)