I use a Lowepro Rover AW Plus and it fits my 4x5 Arca-Swiss F-Line Classic with 90mm Grandagon attatched in the bottom compartment, protected by a Domke Wrap and the focusing cloth. The top portion fits my 6 film holders, a 150mm and 210mm lens (both on those huge Arca-Swiss lensboards) and I sometimes pack my Arca-Swiss monoball and the rest of the telescoping monorail from the F-line in the top as well. I also still have a lot of room left over for more (like my other 6 film holders, water bottle, etc...). The side pockets carry my light meter and focusing loupe.

I used to use a Photo Trekker AW but I found it to be too bulky (though I admit it is a bit more convienent for getting gear more quickly). However, I found that the Rover AW Plus was a much nicer backpack for longer walks since it holds weight a bit closer to my body and it's much more comfortable (though I think the only difference in the harness is that the straps are mesh-lined... other than that it's supposed to be the same as the trekker series harnesses)