I've had my Mountainsmith Bag for 8 Years of near constant use, and it has served me very well. Very versatile in what it can do. It is unobtrusive when running low on gear, yet expands and swallows an impressive amount when needed. The bungee cord is very useful, and the drink holders are easily accessible no matter how you wear it and are useful for storing quick-grab knick knacks.
I use mine while commuting on my bicycle, trail riding, hiking, airline carry-on... it goes with me just about everywhere.
Mine is going strong after 8 years, and I just bought one for my girlfriend as well.
My only, only complaint with the bag was the shoulder strap lacked surface area and padding, but that was easily remedied with an aftermarket strap.
Very good product.