I use a Samsonite shoulder bag, into which I pack 400D & battery grip, 14-70L, 70-200L, hoods for these two, 50 prime, Siggy 105 macro, 2 sets of spare batteries circ polarisers, uv filters, notebook, pen, memory cards and spirit level.
What it doesn't accomodate are 100-400L, macro ring flash (golly that's a bulky little item), monopod/tripod, 30cm reflector, Cokin Z filter kit, any other lenses or provide space for a second body.
So... the samsonite is no longer big enough as a grab bag to carry all my gear. But...
I don't see the point of carrying a camera and it not being ready for action.
I never actually need to carry all of my gear with me.
I tend to want 2 or sometimes 3 lenses with me - typically the 100-400L and the 14-70 or the 105 macro, and come the spring, both. And now the Cokin filter set. And I need a waterproof camera lens cover.
So... my plan is to grab a rucsac, shove in the gear I need today and see how that works before investing in £100-worth of dedicated camera rucsac which would accomodate everything but which would be overkill to carry in terms of the long sunday walk.