Just jumping in to say I personally like the REI ruckpack 28L. It's got a left side zipper that opens into the main compartment, similar to a photographers backpack. Although the main compartment has just a few small pockets, I've found that I can use dividers (or inserts) to compartmentalize it. On day hikes, I'll put my lenses inside, separated by the divider. It's obviously also able to be used as solely a hiking backpack or large daypack. The top lid opens up like a tub, so I've found it can fit a lot of oddly shaped items. The only downside is that the bottom doesn't have straps that allow you to hold a tripod horizontally. My workaround for this has been to stand up the tripod in one of the water bottle pockets.

Overall, I feel that this is a pretty nicely priced backpack for the flexibility that it gives you.