I was using a Crumpler Formal Lounge and had no problems but of course have to put it up in the overhead cabinet. Too big to stuff under the seat.
Do note that some airlines have a restriction on the number of hand carried items. I was able to take the Formal Lounge and a duffel bag up when departing from Singapore but when coming back from Korea, Cathay Pacific only allowed me to carry 1 piece of hand luggage up. Had to check in the other bags.