For big (read: SLR) gear, I do like my Slingshot. But like most photographers I have a great many camera bags, none of which ever quite do the job. My most used camera bags don't even count in the manufacturer's inventory as bags: those are Lowepro camera "pouches" which sling over the shoulder quite nicely and hold a camera, lens, and spare lens very well. In it's usual incarnation, my most oft-carried pouch/bag has an M3 with (collapsed) Elmar-M 50mm/f2.8 + VC II meter, film, a 2-stop ND filter (or the case for it, if the filter is already on the lens), some Tri-X and some Kodak Gold 100 and 200, 2xLR44 batteries (just in case), a lens brush, a microfibre cleaning cloth, a paperback novel (usually, at best, of "airport" quality) and a spare pack of smokes.
I have the same (well, a 2nd) "pouch" with a Canon 5D (original), with 50mm/f1.2 lens, a 24mm/f2.8 lens, spare battery, extra CF card, lens brush and cloth and no room for luxuries like the novel or the smokes.
One or the other is my "go bag" should I leave the house. Usually it's the M3, but far from always. Both are always ready...
...Mike