I have four different "bags" and use all of them at different times. I have a
Lowepro Mini Trekker AW, a
Lowepro Magnum AW, a
Pelican 1520 Case, and a
Lowepro S&F Vest Harness and Deluxe Waistbelt.
When I'm carrying gear a long distance, I love the Mini Trekker. It's a comfortable backpack that's great for carrying gear. The problem with a backpack is that it's a pain to get gear in and out. I either need an assistant or I have to take it off first. Still, if I want to haul a lot of gear a long way, it's my first choice.
When I want to carry a lot of gear a fairly short distance and plan to be in and out of my bag a lot, I use the Magnum. It's a large shoulder bag. I lug it to my shoot location, set it on the ground, open it up, and work from it. It's not comfortable for long distances, but it's bearable. I like the form factor when I need to get in and out of it a lot.
When I'm going places that could be troublesome, I use the Pelican case. It's great for when we are canoeing because it's waterproof. I also use it for travel because I can lock it and lock it to things. If I'm going to be leaving camera gear in a hotel room or car, I use this case. The downside is that it is terrible for carrying. I sometimes use it as a "safe" place for gear and switch gear into other solutions when shooting.
When I want to carry minimal gear, I just use the Deluxe belt. I carry my camera on the hand strap and/or neck strap and carry and extra lens in one pouch on the belt and stuff (filters, memory, etc) in another pouch. When I need more gear, I throw on the vest and add more pods. If you don't mind looking rediculous, you can carry a lot gear this way and have it all accessible. It's not nearly as comfortable as the backpack, but it's much more accessible.
I once had a fanny pack like the one above. I really liked it because it was comfortable to carry and I could rotate it around and work out of it. Sadly, it was stolen. It wouldn't hold my current gear well, so I didn't replace it. If I was only going to use a basic SLR and one or two reasonable size lenses, I'd consider another one. I suppose that it also depends on how much room you have for a pack on your fanny. It's not the sort of thing that fits every photographer body style.