I have traveled with a Graco Argos 70 as free checked baggage on approximately 5 trips in the last two years (three transcons and two transatlantic).

Each time, I went to the checkin desk, checked in my regular luggage and received a tag for the car seat, which the agent attached. I then took it over to the special/large items desk who gladly accepted it.

One tip - the only special packaging that I use is an REI backpack duffel. I take the car seat out of the car, then flick the levers to flatten it out. I then squeeze it into the pack duffel and zip it up. No padding or anything and it's survived very well. They have never checked to see if there was anything else in there.

This is the duffel: http://www.rei.com/product/787296/rei-pack-duffel-bag