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I love my Billingham 335. I use it to hold my 501c, 2 backs, an 80 and 50mm lens, a Leica M6 with its 50mm lens, several lens shades, various filters and a spot meter. It has room to spare for accessories and film. With all the equipment though, its not something I like to carry on my shoulders for long. It is a great bag to take your equipment to a specific location or in the back seat of your car ... and what I do is to set up the camera/lens I want on my tripod with an extra lens/film/filter in a small case or even pockets. Good luck.
Bags I have and use: Think Tank, LowPro, DelseyPro, Billingham, and Kata.
For my Hasselblad 503CW with 50, 80 or100, 150 (or H3D/31 with 50, 100 and 150 + 1.7X) and Leica M8 with 28, 50, 90 ... my choice is the Kata WS 606.
IMO, Kata bags offer the most impact protection verses weight, are easy to access, and have yellow interiors for easy selection in dim light. The WS 606 is a Kata Insert Trolly ready bag with contour shaped shoulder strap and tuck-away wide belt and comes with a "rain-coat".
When traveling I also take a Boda lens bag so I can grab the camera and put a few lenses in the Boda which has all kinds of compartments for accessories, fim, and even a built in CF card wallet.