Where has this been all my life?!  No, not all rolling under-the-seat bags are the same.  This one's greatest benefit is the strap along the back, with a snap in the middle, that allows it to be slung over the handle of a larger bag and snapped in place between the two legs of the larger bag's handle, so that the smaller bag doesn't fall off.  Absolutely wonderful, and not just for airline travel, either.  We just completed a two-week road trip, travelling light and bringing just essentials into the room at night.  My husband's CPAP and heavier clothes meant he needed a regular carry-on size, so he used his rolling carry-on.  I used this smaller bag, which I had just ordered for this very purpose.  Because I could roll both his bag and mine at the same time, he could roll our rolling cooler, and we each still had a hand free for keys, credit-cards, opening doors, what have you.  It meant we could make do with only one trip from car to motel each night and back in the morning, instead of two -- and in this heat, that sure was a godsend.

OK, there's still another big benefit:  It's that, in addition to the main compartment and a zip compartment in the front (which was all my previous rolling tote had), this one also has a thin but complete compartment along the back that zips open separately, much the way some fancy attaches have a front for business and a separately opening back for clothes.  If you stuff the front, the back will be constrained, so it won't hold too much -- but it did hold a large-sized Rick Steves pack-it container for men's shirts.  Amazing to have so much storage space.

My husband is now envious.  For airline travel, he's going to order his own one of these, so that he can have both a regular carry-on to check at the gate for small planes and a smaller roll-on they'll let him take aboard, neither of which he'll ever have to carry.  I'm going to use this one with my regular rolling carry-on for the same purpose.  A lovely way to travel light, now that we're all avoiding checking luggage whenever possible, and still saving shoulders and keeping a hand free.

Oh, and yes, this rolls like a dream whenever you are rolling it on its own.  A real winner.