To my husband's dismay, I'm planning on gate checking our carseat. (Since he does not want a separate seat for our son -- against my better judgement). I've checked my seat as luggage and they had the thick plastic bag. This was totally fine with me at the time, it came back perfectly clean. However, I've heard of cases where damage is done to the carseat during handling. Especially when it is unloaded and drops off the baggage carousel. This could potentially cause minor cracks in the seat. If you notice any cracking -- no matter how minor -- do NOT use the seat. It could fail horribly in a crash. If you do check the seat as luggage, examine it carefully (the plastic part, so you might need to take off the cover to do it properly). Even if you don't use the carseat on the plane, you can gate check it. This would minimize the handling of the seat and avoid the drop onto the baggage carousel. As an added bonus, if there is an empty seat, you could install it on the plane.
Have a wonderful trip,
Jennifer