Probably too late to change your mind, but FWIW I've used a Lowepro Fastpack 350 for a few years now.
It's a comfy well-priced backpack (around £50 or so, maybe less) - doesn't scream "expensive cameras inside, please mug me" - it has an easy-access bottom compartment in which I keep 5DMk2 with attached 70-200mm lens, and a 16-35mm and 24-70mm and there's room for my Sekonic lighmeter in there as well. Well padded compartment with flexibility to move the padded dividers to customise your space.
In the separate reasonable sized top section, I keep a drink if necessary, plus a flashgun or two, a grey card and Color Checker Passport, together with spare batteries, lens brush, spare CF cards (there are special pockets for these), pen, 1.4x Extender, sometimes my 50mm f1.4 and all sorts of little useful bits, cables, wires, chargers, etc etc.
I've really crammed some stuff in there on many occasions and the zips have stood up wonderfully to some serious stress. Very impressed with the manufacturing quality.
In the front pocket I keep other interesting small items (my passport and airline tickets if I'm taking it as hand luggage) and other stuff. It struggles to fit in small aircraft overhead lockers (or more precisely, when it's fully laden, I struggle to lift it up there!) but it does fit easily underneath the seat.
In the shallow pocket in front of the bag, I keep a black bin liner to kneel on wet ground or whatever if the circumstances dictate, and a shower cap for the camera in the rain (until I get a 1DX of course!) and a spare credit card and some coins and other flat items.
There is also a separate external mesh pocket in which you can put a Coke bottle or maps (on a dry day) or whatever.
It's not easy to get a tripod attached as there are no dedicated straps as such, but it is possible in a couple of different inelegant ways.
Finally, it has a separate compartment for my 17" MacBookPro laptop. That makes it a bit of a heavy brute, but it's handy to transport and keep them with you when you need to, but you'd leave the laptop at home if you were fell walking or whatever.
It's comfortable to wear - two normal fully adjustable shoulder straps and it has adjustable hip straps too (which I've never used, although I've tried them and they are certainly comfy) - and has been very hardwearing. It's had some stick. I can't fault it to be honest. It would be nice if it was fully waterproof I guess, but it isn't ... it's certainly shower proof - I've never had any water ingress - but I wouldn't like to be out in any downpour for any length of time with my cameras and laptop in there. You can apparently get separate waterproof covers for it if you want - I've never bothered. You could also use a pedal bin liner inside the top compartment too, if you wanted to be uber-careful in protecting the contents, but I haven't done that either, nor needed to in the last 3 or 4 years I've used it.
It does all I've ever needed, and as you can tell, maybe just a little more besides!
Here's a typical link ...
http://www.amazon.co.uk/Lowepro-Fas...2?s=electronics&ie=UTF8&qid=1350026444&sr=1-2
