I use the CARES harness. I would never fly without it until my son is much older. I've baggage-checked the carseat in a bag many times. The bag has holes, but the carseat has never seen any damage. I guess we've been lucky in that regard.
Car seats that are checked at the gate are subjected to exactly the same treatment.
Nope- you can see them wheel it down and put it on a cart, then into the compartment. They're not inside throwing them around. Sometimes, when her carseat was the only thing checked at the gate, they just pulled it in and put it in an overhead compartment
It can also be rough at the gate checkin. We had a stroller checked in at the gate, I could not use it anymore when we arrived. This was also the only time I was traveling alone :-| not cool. I could still ride it on 1 of the 4 wheels. Luckely my dad picked us up and he could bend it a bit into shape (the guys from the plane couldn't ).