Most people on here advise never to use the rental car seats. They are old, dirty and you have no idea of how safe they are. I would either bring my own or a lot of people suggest stopping at
walmart to get a cheap seat to use there. We have been travelling with small kids for years and we curbside check our carseats (in a bag) and never have had a problem. It is really only a pain to get them from baggage claim to the rental car, so we just rent one of those big carts or tip a porter. Well worth the $5-$10 over the exhorbitant costs to rent a dirty, unsafe seat.