I JUST had this conversation with my mom this morning. My son and I are taking a vacation with her this spring and she was saying "we can just rent a carseat with the rental car."
I tried only once to use the rental seat when my son was an infant. The seat they provided was only a 3-point harness (who knew they still made those). He road in it for about an hour while I drove to Target, bought a new seat, and drove back to the rental company. The seats you can rent are not ones you would EVER buy. They do meet minimum safety standards, but only the minimums. My son is 4 and almost 40lbs, so the rental seat would be a booster only, but i still harness him (not to mention he'd be in a seat that other kids have potentially thrown up on, peed in etc).
That said I'll be bringing his own seat along. Also, I've found that plane flights are much easier with a child in a seat he's used to. I plan to bring his seat on board with me, so while there is that lugging-through-the-airport to deal with, it'll make the flight oh so much easier.
For $12 at Ross I bought a carseat duffel back a couple of years ago. You put the seat in it, and it has wheels on the bottom, so it's easy to just pull it through the airport as an extra piece of luggage... and I'll check our bags on this trip so I don't have all of that to pull along.
Hope this helps,
T.