Well after i picked up the 70-200 2.8 IS i was in need of a new bag. I went with this same bag you recommended. Bought it yesterday and used it pretty much for 12 hours at the Harley Davidson 105th anniversary in Milwaukee.
Here's what i like.
1. My gear is safe in this bag. Well padded, i like the extra dual bag inside that offers a layer of protection. This should also work great in bad weather conditions.
2. Plenty of room for all the gear i have. Pretty much the same rig as you have, except i'm still shooting an XT and not a 40D. I dont have a flash but i also have a 50mm prime. Plenty of inside pockets for keeping organized too.
What i dont like.
1. Not very comfortable. I dont know if i like the way the straps are with the rubbery stuff on them. Obviously they help keep the bag on your back and not slide off, sometimes it would be nicer if it did slide off a bit easier. I also thought the back pad could be better.
2. I miss my shoulder bag being able to not take the bag off or set it down and being able to change gear.
That said. I think for the price I am happy. I didnt want to spend a whole lot more and now i have a true camera bag. My other bag was a simple northface hipsack converted into a camera bag.