So I have been moved slowly, and with great reluctance, to rolling bags. Around the time T. was born, I bought a Victorinox roller that could also be used as a backpack and had an attachable day back (I had long ago sent the Eagle Creek version of same along to live a long and happy life with a friend). Turns out I _hate_ monopoles. I bought a silver rolling Delsey that I really liked, but then I loaned it to my mother-in-law and urged her to keep it when she really loved it. I bought a second rolling Delsey (purple!) and have been using it since. That's the bigger of the two bags in the photos below. I've had it for a while and don't intend to replace it any time soon. It is a bag to check, never to carry on, but there is zero risk that it will ever be treated as an oversize bag because it is well within the limits.
When my friend K. came to visit a few years back, she had a Zuca. Mmmm. Of course I had to have one. You can sit on it (really -- like 300 pound weight limit. You can even have a small child in your lap!)! I almost managed to roll the purple bag _on top_ of the Zuca, but that didn't quite work. The Zuca always fits in the overhead, but never, ever, ever under the seat. It came with packing cubes. I love it. I don't _always_ travel with it, but I really like it when I do. On the carry on only short trip, the Zuca supported my black Pathfinder bag (which, if you don't overpack it, fits under some seats) and A.'s pink Hello Kitty case.
The Pathfinder bag dates from when I was scuba diving. It sat on top of the monster, hard sided case that held the suit and fins and all the rest of my vacation clothing. The Pathfinder was big enough for whatever I'd need on the plane, a change of clothes, my regulator, prescription face mask, etc. I got rid of the hard sided case long ago (it was always at risk of an oversize charge), but the Pathfinder has survived due to its ability to (a) play nice with the handles of rolling bags and (b) willingness to scrunch down and fit under airline seats.
But the Pathfinder is too wide. It's 18" inches, so while the Zuca scoots down the aisle on the plane with politeness and grace, the Pathfinder feels compelled to touch both sides and grab at people's coats and so forth. Not nice! Pathfinder is awesome luggage; what I have seems to be a 15 years earlier version of this: http://www.pathfinderluggage.com/Product.aspx?PROD_CD=P3157-01-18TT, only they've figured out to smooth out the side pockets and they advertise it as overhead only. There is little evidence of the decade and a half of (ab)use this thing has seen.
I went in search of another option, and came up with this, which honestly seems kinda silly.
It will hold electronics up to a laptop in a padded sleeve. It is big enough for a change of clothes and snacks. It has a good sized water bottle thingie on the side. It rolls, but plays nicely with rolling bags. It is willing to pretend to be an oversized handbag.
It is almost but not quite as _tall_ as the Pathfinder is wide. But it is not wide. A little wider than the Zuca, but plenty narrow for sliding down the aisle. And people swear it goes under airline seats and I believe them.
I'm not sure, in practice, what I'm going to do with this thing. Will it really ride on the Delsey to the checked counter, and then roll alone? Will I stack it full of other crap (but then where will the other crap go while I approach the counter, since when it is stacked on the Delsey, there isn't any handle space leftover?)?
In any event, I feel great optimism. I bought a silly looking sherpani backpack once, and used it hiking for a few years; it was amazing. I wasn't as much of a fan of their string wallets, but you win some, you lose some. Here are pictures of the Delsey and the Sherpani. I left the Sherpani open so you could get a sense of the volume and also see the interior lining:
And the Zuca and Sherpani: