I have this bag too and I really like it. When solidly packed it can carry a lot of gear. Earlier this year I travelled with a three-lens Hasselblad kit (50, 80, 150mm), a three-lens EOS-1v kit, and a Wollensak Stereo 10, all in the Flipside 400. There was no room left for any significant accessories like flash units, etc, although I could still carry smaller stuff like filters, meter, film, etc. Although it carried all this gear the weight was something I'd rather not experience again for extended periods like that! Since I've been back I've been using the backpack as a one-camera kit bag and it's great. You'd have no trouble putting your kit in it I'm sure. One thing that particularly drew me to it, especially for travelling, was how the main compartment is accessed from the side that contacts your back. That's much more secure in my opinion. No low-life in a crowded train can unzip the backpack and steal a lens on the sly.
Originally Posted by JLP
Having said all that, the only other backpack I own and can compare it to is a Micro-Trekker 200. It can fit my 3-lens Hassy kit with a few small accessories, but it would be too small for a four-lens kit.
The pouch in the top left of the photo is my light meter and the camera at the top of the photo is a Stereo Realist (I ended up taking the Wollensak Stereo 10 on the trip rather than this Realist though). The main flap has some storage space on the inside for thin items, and on the thief-accessible outside rear there is a larger storage space as well for miscellaneous items. Ignore the Canon gear and the Stereo Realist and you'll get an idea of what else you can fit in this backpack on top of my 3-lens 500C/M kit.