We did Oasis in the fall of 2010 and LOVED it. I will echo what everyone else said about it never feeling crowded (albeit during parades in the promenade) and there being so much to do!
We originally were booked for the Allure this coming fall, but since our son will only be 13 months old, we decided to go with Carnival since they offer a much better rate for him. So for me, the only con is one with RC, for not giving a 3rd passenger a nice reduced rate. I am not going to pay $1000 for my one year old who isn't even allowed to access everything the cruise has to offer.
Oh, I will also say, the con on our cruise was the stop in Costa Maya. They really need to build this port up. We would have rather stayed on the ship.