We always rent when we fly in. It might not be the cheapest option but we're not interested in dragging the large variety of car seats we use around the airport. I'm not sure if we have saved any money though - the car seats are not cheap to rent from the car rental agencies, we have two kids and we fly a couple of times a year, so you can do the math and decide if some kind of rolly-travel seat might be cheaper.
If you have friends or relatives in town they can meet you at the airport with a used car seat or one picked up on Craigslist.
The rental car kidseats usually come in a variety of sizes and you just drag it out of a closet they keep in the covered parking. Much easier than transporting your own in my opinion. Downside is cost and also the cushioning is usually not as plush as one you might purchase yourself. I would not trust the airline to pamper our car seat if it were checked.

3 and 5 yr old should be able to sit in a seat without the harness. We've found the Cosco Scenera easy to travel with (probably cost less to buy then renting a car seat )

Always bring seats. Either bring on board or gate check depending on which seats. Try the Evenflo Securekid or or get ahold of a Safety First Go Hybrid.

Anonymous wrote:I've rented a car seat a few times. It's never had poop or vomit (seriously?) on it. I wouldn't check my car seat because, as a PP said, I would be afraid it would be damaged by the baggage handlers.

But your fine using a seat you have no idea of the history of fit your kids? Crazy. We bring our seats and use them on the plane. I don't trust the rental ones.
The bubble bum is awesome for travel once your kids are booster ready.

I've rented a car seat a few times. It's never had poop or vomit (seriously?) on it. I wouldn't check my car seat because, as a PP said, I would be afraid it would be damaged by the baggage handlers.

We have rented cars seats many times and the seats are basic but fine. They have never been dirty ir seemed unsafe. It is much easier.