I use this bag when I need to carry more kit than I can accommodate in my flipside 400. Downside is it is quite bulky.
Very useful features of these bags are:
a) Place on the ground and go through the back for the main compartment so your cloths remain clean and dry.
b) Two straps on top for securing additional clothing layers.
c) Large pouch on the opposite side from the main compartment for odds and ends and small items like hats, gloves, reflectors, batteries.