I haven't used the CARES yet. I actually am still bringing my son as a lap child, so I can't bring the car seat on board anyway. But I am still trying to figure out if I should bring it for the car when we actually get there - for when we are picked up at the airport.
The other thing is that at least my Britax Boulevard is pretty big, and I'm not even sure if it would fit in a regular airplane seat. I had a tough enough time with my Graco Snugride (even had a flight attendant once ask me if using the seat was 'allowed' on board...um, you should know that it is!). I just can't see it fitting even though it is approved for airplane use. I bought a big backpack-like thing so that I could carry the car seat on my back through the airport, but it just seems like such a hassle to carry it anywhere that I'm not sure I'll even use it.
I think the restraint part of the car seat is attractive, but again, not sure how happy my guy would be sitting in it for too long, and there's just so little space that I feel like he'd be happier having the extra room and me making him a little bed with lots of blankets/pillows. Not sure. But I only have one, so I can see why you might want it with two little ones!