I also have good experiences with the Lowepro Fastpack 350. It has enough space for a camera and at least 3 lenses. I used to have a 5D2 with 24-105 attached, a 70-200 F4L IS, a 100mm L macro and a 50mm f1.4 in the camera-compartment. That's about the most you can get into it. This compartment is in the lower half of the backpack. The camera-compartment opens at the side and the front(viewing from behind) and so it's quite easy to leave one strap on your shoulder and quickly get to your camera lenses on the side. Because the camera-compartment is on in the lower half of the backpack you don't have to worry about the weather.
The top half of the backpack has plenty of space for things you would need like supplies, batteries, filters.
The pack also has place for a good sized notebook/laptop or papers. This compartment is basically flat against your back.

From what I understood, the backback is "weathersealed". I think the zippers are dustproof, but I'm not sure if the backpack is up to massive amounts of rain. My camera at least never got wet, because it's safe on the bottom.

Hope this helps a bit.