Updated May 6, 2006
Like many people I've found that there's no one camera bag that works well in all situations - so I have a few bags that I use in different situations.
The BackpackA backpack is nice for times when you need to carry a lot of camera equipment. It's not so nice when you need to use the equipment. I use a backpack to store most of my equipment and carry it while traveling. Sometimes I'll use it when I know I'm going to a specific location to shoot. I tend to favor the Lowepro Trekker line.
The Shoulder BagShoulder bags are easy to work out of, put down, and pick up. You can get at their contents without taking the bag off and putting it down on the ground. I use a Domke F6 bag for events, and other situations when I don't mind carrying a camera, 3 or 4 lenses, flash, and other stuff on my shoulder. As long as the camera and one lens is in my hands rather than in the bag the weight is comfortable.
The Small Shoulder BagThere are times when I just want to take a camera and a lens or two. Day trips into the city are a good example. In these situations I want a bag no larger than necessary so it's not in the way. I use a small canvas shoulder bag that I lined with velcro and add a divider from one of my other bags. It holds it's shape well and is very easy to get the camera into and out of the bag when walking around. The biggest negative is that it kind of looks like a woman's purse.
The BeltbackI resisted getting a beltpack (or waistpack) for a long time. I thought wearing one would make me look like a dork. Then I got the Lowepro Off Trail 2. It makes me look like a dork, but when I'm doing a lot of walking it's the most comfortable way to carry my equipment and have it easily accesible. When hiking it allows me to wear a daypack that can hold extra layers of clothing, food, and water. It's become my primary nature photography bag.