I've had a Lowepro Orion Trekker II for a while now and I'm really happy with it.
I wanted a backpack so I could take my gear with me everywhere I go and I also wanted a bag with general storage space so I wouldn't have to carry another bag with me.
The bag has 2 main compartments. A top one for general storage and another one for the camera and lenses.
Interior Dimensions (Main): 11W x 6.5D x 6.3H / 28 x 16.5 x 16 cm
Interior Dimensions (Top): 11.8W x 6.9D x 10.4H in / 30 x 17.5 x 26.5 cm
On the outside, there are 2 pockets with zippers to put all kind of things you need fast access to. I wouldn't put valuable things there, though, since they would be way too easy to access...
The backpack has 2 straps to support the weight while hiking: a small strap to go around your chest and bigger one that goes around your waist. Both are adjustable and removable in case you don't like too many straps dangling from your backpack.
The backpack is very comfortable even in long hikes.
The top compartment has 2 small pockets for memory cards. They're made from a transparent nylon material covered with netting so dust or liquid can't get in. Plus, there is a big storage place (It goes all the way behind the two memory card pockets) with a zipper for an MP3 player and a headphone cable port so you could listen to music even when the bag is closed and on your back.
Behind those 3 pockets, there is a big netted pocket where you could put things you don't want to move too much in the main compartment.
However, I wish Lowepro made the main compartment just a little bit bigger so an A4-sized magazine would fit inside (It barely does.) The bag could fit a thick book with ease, since I used to carry a one thousand-paged book in it. It just couldn't fit anything bigger than legal paper.
Here is all my gear inside including my 350D with grip and EF 28-135 IS USM attached:
The lower compartment has a removable insert. I guess that's good when you want to clean your backpack since it comes out as one unit.
One of the things i like about the bottom compartment is that you can actually leave the zippers open while carrying it on your back without the lenses falling out of the bag. So if you need to change lenses fast, you can do that without having to take the bag off your back and without having to mess with the zippers.
(Pictures taken with Canon powershot S200. Sorry for the poor quality, I was rather lazy to take proper shots! I hope this isn't too painful for your eyes