I use the canon 200 EG backpack. http://www.bhphotovideo.com/bnh/controller/home?O=&sku=248295&gclid=CMSTqcDlg74CFSISMwodhTUA8g&Q=&is=REG&A=details
Granted - it's the first and only camera backpack I've ever owned, but I haven't come up with a complaint yet. Others may have reasons not to buy it. But for $40 or so (I think right now at B&H it's $39), I can live with it being a second pack if I find one in the future that I like more.
I carry a 6D w/ 100-400L mounted, a 40mm pancake, 17-40, a 100mm macro and the 24-105 kit lens in that pack as well as a 430EX II flash, batteries, charger etc and some filters in the pockets as well as all of the cables etc. The only thing I haven't figured out how to fit in there is my flash diffuser (fong - not super compact). I still have room for a few smaller accessories (Batteries, filters, caps/covers/rain covers etc). I usually throw a pack of sunflower seeds or something like that in the outter webbing as well - can't go hungry, right?
The only thing I think I wish it had was a water bottle holder and a dedicated way to add a tripod case to it in a built-in sort of way... I use velcro straps onto the side, which seems to work fine. It might be nice to get a pocket for a tablet in there too, but I can live without it... more capacity means more stuff I'll put in there, it gets heavy enough as is. I added a piece of mesh material - similar to a laundry sack, to the inside to cover the lens holder area so that in the event that the zipper comes undone (never even come close to happening so far - I'm just a little paranoid), that there's one more layer of protection against lenses falling out or rattling around.
My main use for it is to carry my gear to remote locations on hiking trails, tops of hills/mountains/cliffs, along rivers etc. Not really urban photography.
One note - the bag had a sort of foam-plastic odor when brand-new, but it dissipated within a few days of taking it out of its plastic wrap and I haven't notcied it at all since.
Just my 2 cents.