The American Academy of Pediatrics recommends it. The airlines recommend it.
My wife and I decided against it. We didn't believe it to add a substantial amount of safety, compared with the bother of it all.
However, this is a personal decision that every parent must make. My wife, a Fellow of the American Academy of Pediatrics, wouldn't recommend to her patients that they DON'T use the car seat, but if they asked her as a "Mom" she'd tell them what she decided and why.
Our then-2.5 year old did just great when we went in May 2005.