I personally carry MountainSmith lumbar packs. My daily carry is the Day version, and I love it. I have carried the same bag for 6 years, and it is still going strong. Color might have faded a little, and the yellow zipper pulls look a little dirty, and the rubber coating on the ends of one zipper came off. Nothing to stop using it though. Its not waterproof, but I haven't been afraid of it getting wet when walking a short distance in the rain. It is also small enough that I can clip the waist belt and keep it close to my body under a jacket.
It has some pull straps to make it smaller, or release to hold more. This is my goto carryon on a plane, or really any travel. It will probably be a diaper bag when a baby comes along too. Though its not the best design for that.