Got one off ebay. It carries more than my current Tenba. I too would not carry everything. It gets heavy. I put a 20d with attached lens up to 70-200 2.8 sideways just like the picture above. If I take the 70-200 off, I can store that on one of the sides. Also in there are 85 1.8, 17-40, 580ex with stofen attached, a canon hv20 videocam on top. extra 20d battery, 2 extra batteries for the camcorder and a couple tapes, and some negligible (in terms of weight) accessories. This is comfortable. If I add the 24-105 and the 50 1.4, then it gets heavy. I will use it to carry everything to my destination, then leave 2 lenses at the hotel or what have you to lighten up. It almost looks like you can use the 2 rubbery straps on the outside to attach a monopod, but I dont own one so no joy. Using the waist belt helps a lot but the top starts to sag a little. Adjustment might help.
2 bodies with max 3 lenses? Sure I don't see why not. I cannot change lenses without taking it off, so the sling advantage seems to only come in when storing/removing the camera itself with lens attached. Maybe some more practice.