As a petite female living in a small NYC apartment, light weight luggage with a space saving option was very important to me.
I have used it for around a dozen trips, ground and air travel. No stains, no rips, no tears. No damage to the items inside.
It folds really well and I can easily store it under a bed or in a closet. The side pockets are very handy, and the bottom support is very sturdy.
The most important thing to consider while buying a softside luggage piece is that shoes, cosmetics, etc. must be stored in a hardside/hardshell packaging. For the shoes, I simply use a shoebox. For the cosmetics, perfumes or anything fragile, I used a sturdy/hardside cosmetic case.
In terms of capacity, I have used it for weekend trips as well as a full week's trip. It depends upon what you carry, and how you pack. I recommend Eagle Creek's travel gear pack-it cubes to keep everything compressed, organized and safe.

Edit January 2013- One of the bases has come off leaving a small hole on the underside of the duffel. It is not waterproof either.