We stopped using the CSR in the air when the girls were about 3. By this time the benefits of the CSR decrease substantially. (I am talking air travel not car, just ftr) Three's are usually old enough to understand they need to sit and leave the lap belt on. By 3ish they are usualy uncomfortable in the CSR on the plane vs the car and as others have mentioned, too close to the seat in front. Mine loved to color (try color wonder!) so having the tray in front was a lifesaver - most CSR's will prevent the child from using the tray.
For those with 2's and small 3's that want to use the CSR, make sure it is approved for airline use, is not to wide for the seat, usually 17-18 inches and has an integrated harness. Booster seats without the harness are never allowed.
For seat kickers, have one parent sit in front of the "kicking" child. My dh and older dd would sit in the row in front of us so dd was in back of dh. We then switched for the return trip. This way if you have a kicker they are kicking mom or dad and not ruining someone elses day. We also removed shoes to lessen the blow!
If you do decide not to use the CSR in flight BRU has several styles of great bags to use when checking the CSR. My sil has one that can be used as a backpack or even rolled via handle, I wish they had one when my dd's were younger!
TJ