We used them for the last two trips we took (DS was 4 months and 9 months). We ordered diapers, wipes, shampoo, take & toss cups, formula, throw away bibs, diaper sacks, pretty much anything we could so we didn't have to pack so much - and MY, was it a lifesaver!! The box was waiting for us in Bell Services when we arrived. It saved us from bringing at least one extra suitcase (and you know how they are charging for bags now!).
I am doing it again for our trip in December and would HIGHLY recommend using diapers.com. Free shipping is always a plus - and it's too easy to spend $49 anyways on diapers and wipes alone. Good luck!
Another hint - We also rented toys and a baby gate from A Baby's Best Friend. That was waiting for us at Bell Services on check in too and provided us with all new (and exciting) toys for Kaden to play with. All we had to bring with us were toys for the plane. Hope that helps!