It is not cheap (about $70). It is small and light, and if your traveling partner is an engineer, you'll probably get it deployed correctly by your second flight. It probably does not make sense to buy this for a single trip, and practicing using it ahead of time (maybe on a chair?) might help a lot to get it right the first time. All that said, OMG! This is the best thing ever.
I think I've mentioned the light part. Anyone who has traveled with a baby bucket or car seat is familiar with the following problems. (1) The seat has to go in the window seat, which means the adults get to duke it out over the already uncomfortable middle seat which has become a sophisticated torture device by adding the plastic seat with its plastic edge that digs into you no matter what you do. (2) You have to transport the seat through the airport (bad enough), specifically, through security. To be fair, with the new line at some (most?) airports for families and people with special needs, you have a reasonable shot at beating the casual traveler through security these days if you pack correctly. But still.
There are a variety of devices for transporting that seat. (1) Stroller it if the kid is walking (maybe hang it off the stroller) or being carried. (2) Buy a device to wear it on your back (but then you can't wear a backpack). (3) Buy a device (I recommend this option) called the Traveling Toddler or some such to attach the carseat to a rolling carryon (make sure you get one that it fits on) and use the combination carseat-device-carryon as a stroller if you just need one part of the time. (4) Buy a device (Go Go Kidz) to attach the carseat to what is essentially a luggage cart on steroids and use the combination as a stroller (or luggage cart). Some of these options work better than others for any given family/luggage situation.
The harness enables you to contemplate two possibilities. (1) You can check (either regular or at the gate) your car seat. If you gate check, you still have to get it through the airport. (2) You can rent a carseat with your rental car, ship your carseat or otherwise arrange for a carseat at your destination, thus dodging the entire carseat transport issue. We tried the gate check option, and it got checked through, so on the return, we checked it regular. Because there are straps and such on a carseat that are prone to getting caught, we bought a bag for the carseat (it has handles, but is not padded), which was another $20 and worked well. There are parents who object to checking carseats on the theory that what baggage handlers do to luggage constitutes the equivalent of a car crash and therefore impacts the usability/safety/whathaveyou of the carseat.
To which I say, hey. Your call.
I love me some CARES. Because T. is a 50 percentile baby height and weight, I expect to get about another year of use out of it with him, and then A. (baby on the way) will have her shot at it. I'd be willing to consider loaning it out to a friend who was traveling in the interim, and expect that when we're done with it, there will be a lively used market (assuming there hasn't been a recall in the meantime) for occasionally used CARES harnesses.