There are many different types of backpacks and you might want to specify what your needs are.
Whether you go with a Thinktank or a Lowenpro or maybe skip a "photo" backpack and create a custom backpack designed for your needs. Some people opt for more of a "sling" bag to get fast access to equipment rather than having to take off their backpack to get to the equipment. Again, you might want to list your "must haves" and "needs" so people can offer up their experience with backpacks.