I have a ton of bags, so it seems like every time I travel, I take a different carry-on (btw, I mean the carry-on I keep in the seat with me, I still haven't mastered the art of carrying on all my luggage and not checking - but I am trying to get better). I am a notorious over-packer and never bring a little carry-on or just my purse. It just can't be done. I need something that fits my abundance of magazines (only two of which I probably get though), my book (which I may or may not read), my makeup (what if my luggage is lost, I need my makeup!), my purse (who wants to carry two bags - my purse needs to fit inside my carry-on) and whatever other little things I throw in (iPod, pashmina/wrap, snacks, etc).
On my most recent trip I brought my Longchamp - Le Pliage Expandable Duffle. It's a great bag that folds up into a flat little compact square, opens to a good sized bag when zipped and also expands 6" vertically into a larger bag. On the plane I used it zipped and fit everything perfectly. When at my destination I used it expanded for an overnight trip and it was large enough to fit everything I needed. My only complaint is that the inside pocket is quite small and doesn't close or have a zipper.