I carry my 450D + grip fitted with my 70-210 lens with room for the 18-55 and a nifty fifty [all with hoods] or flashgun in the lower compartment.
I try to keep the top section clear for clothing and food etc when out walking but there's lots of room in the top section if you wanted to fill it with photo equipment.
The main flaps have zipped compartments that I keep a remote, grey cards, lenspen and a couple of filters in, plus there's a couple of velcro tabs for extra SD cards.
Add to that a tripod carrier, space for a water bottle and a waterproof overcover and you have a pretty versatile bag for being out and about in for your camera and other gear.