What sort of redundancies are built into the quads?
None really. The more expensive models that have GPS have a "
return to home" setting that if they lose RF they will go to a specific altitude, fly to home, and land. Others have a function that does a lazy turn back towards
base if they lose RF. They do not have sensors in them to avoid obsticles or anything like that. You have to pilot it. If you break a prop, a motor dies, or your battery goes the thing will just fall out of the sky. They are not like RC planes that will glide down. They fall... and they can be rather heavy. There is also the issue that they are driven by Lipo batteries that can be rather explosive if hit right.
The one I have right now is pretty small. It is a 50 dollar Chinese thing that can't really hurt you. It weights 3oz and you can stop a prop at full speed with your hands. I'm building one that is much heavier and much more powerful. If you get in a fight with it you will lose every time. This is why I don't fly it around people. These things should not be flown over crowds. It is actually pretty hard to judge depth when they get down range... It would not be out of the question for a drunk person in the lawn the grab one out of the sky. As you can see above, they can cause some serious damage.
However, they are amazingly fun to
play with. I'm kind of annoyed I did not have one as a kid. They are very fun in empty theatres. Noticed I said empty.
If you are at all interested in them, buy this:
http://www.amazon.com/dp/B00K5Y22VG/?tag=controlbooth-20
Get a set of batteries to go with it:
http://www.amazon.com/dp/B00LK0DY3O/?tag=controlbooth-20
What I don't want to see is these things get regulated because people are idiots with them. You can't fly them in federal parks because some guy crashed one into a mud pot in Yellowstone. You can't fly them in DC because people keep flying them at the white
house. They make stories way to often and the news reports rarely mention the "piloted" part. They mention the "drone" part and never mention that someone actually had to drive it there.