I just tried out the K3L, loaded it up with 2 bodies with lenses attached and 2 additional lenses. They all fit in the camera compartment in the bottom. My HP 6700 series laptop 15.4" widescreen fits nicely in the laptop compartment. Weighed in at 20 lbs without any accessories, probably get up to 25 lbs with extra batteries and a couple bottles of water.
It's extrememly comfortable, very well balanced, doesn't feel like 20 lbs on your back. The back has a little system to keep the bag from touching your back so air flows through and you don't sweat. It makes the bag stick out a little more, but it's very comfortable. Nice waist strap and front strap to balance out the bag perfectly.
It's a fairly big bag, but if you want to carry a laptop, a bunch of camera gear and a jacket, water, tripod, and accessories, it's a great bag. If you don't need the laptop compartment, the Alpha will hold a little less, but is smaller.