I use a Nimbus Latitude by Granite Gear, an ultralight pack. The pros are weight, comfort, and the full frontal zip feature. A lot of packs come with only a top entry pack. The larger packs will come with a "sleeping bag" access zipper at the bottom of the pack, but I liked this because I can access anything in my pack at anytime, without having to take everything out to get to something in the bottom.
There are two cons for this bag. It is not fully rainproof. The center is neoprene, which is water-resistant, but rain has a chance of entering the pack if you don't have the cover accessory that granite gear also sells for around 10 bucks. The other con is the lack of outer pockets. It only has two, and if you fill out your pack, it is difficult to use the zippers for anything other than a camera, or snacks. Granite gear tends not to use outer pockets, instead using additionals, such as camera cases, and waterbottle cases that fit on the waist belt and the shoulder straps. But if you're going lightweight, you don't need the extra pockets.
