I upgraded to the Lowepro Flipside AW400 when my gear outgrew my old backpack (actually, I initially bought a different bag and returned it because it was uncomfortable to wear for more than five minutes). There are two big things I love about the Flipside: the comfort, and the convenience. The Flipside doesn't skimp on the shoulder padding. The waist-strap is also well-padded. With the waist strap secured, most of the weight of the bag rests on the hips, making it much more comfortable to wear. I wore it through an entire wedding with only minimal soreness afterwards.
But the Flipside is also very convenient. Because the backpack is a back-entry style, while wearing the waist strap, you can remove the shoulder straps, swing it around in front, access your gear then put it back on again--all without removing the pack or sitting it on the ground. That makes lens swaps much easier to do, not to mention faster. That gives you the quick access of a sling bag with the larger capacity of a backpack. The back entry does take some getting used to, but it is also more secure, since no one can open the pack while you are wearing it in a crowd. The bag is also well organized, with a tripod strap and rain cover is included. The AW400 will fit up to 6 lenses.