The best ones are made of cordura, reinforced with double-stitching and bar tacks at the stress-points, have a waterproof lining and are large enough to offer some flexibility in what you choose to bring on any given trip.
It meets are the criteria listed above, but when you fill it to capacity it becomes less comfortable to carry due to the "yoke" system used for the shoulder straps.
Plus, in recent trips where I've had to be gone for up to a week at a time, it simply won't hold enough to get me through a week of classes, workshops, services and shows unless I want to wear the same three shirts, one pair each of slacks and shoes the whole trip and still have room for multiple books, music, teaching accessories, cords and laptop.
So I've been casting about for an even larger backpack that's messenger tough, yet sized so I can sneak it aboard as carry-on in addition to my guitar (which MUST be carry-on).
For the last several months, I've been taking a hard look at Chrome's Warsaw II messenger backpack. This thing is built to serve professional bike messengers, with a shape and [flat volume] capacity of 24" x 25" 6" (more if you expand the compression straps and stuff the outer pockets and "hidden" compartment, too), allowing them to transport even huge cardboard boxes by bike. I'm not ever going to transport anything that large, and certainly never on a bicycle -- those days are happily far behind me now -- but the capacity means it could be the ticket for those week-long road trips where I'd like to get away without needing a checked bag.
This week, I scored one used for roughly half of what a new one would cost, with free shipping, online. It arrived today, and when I sat the two backpacks side-by-side, I realized that, even with the loss of some top-loading capacity (The Warsaw is not a rolltop, but has a long buckled front flap), the Warsaw II still has more capacity for the thigns I'm likely to carry. Sneaking it past a ticket agent might be a little harder -- the bag is not as slim-lined and has squared sides and bottom -- but I might be able to make it work because when it's all closed up it's height is a little shorter than that of the Ivan. It seems worth the risk. So I 'll use it on the first of my fall and winter mini-tours this year, and I'll report back with a comparison. (And after I do, one of these two bags will be for sale.)