I have used AmeriBag Healthy Back Bags almost exclusively for at least the past 20 years.  My current favorite is the Small size in Distressed Nylon - Khaki.  However, over the years I have also used the Leather and Microfiber models.  The other reviews do a great job of detailing what an awesome bag this is.  I just wanted to add the following:

1)  Calling this bag Small is somewhat misleading.  I do NOT travel light, but the Small size is quite adequate to carry everything I need.  My gear tends to expand to fill up allotted space.  If I carried a Medium I am quite sure I could fill it with 20 pounds worth of stuff.  In my experience the Small fits nicely under a normal airline seat along with my briefcase.
2)  The Distressed Nylon bag machine-washes beautifully.  I even machine dried it - air only, no heat.  I was very pleased with the result.
3)  Distressed Nylon is more durable than it sounds.  I have used this bag pretty much daily for over a year, and after a wash it looks brand new.

This bag could change your life.  Try just one, you will want more in other styles and colors.