Camera bags are very important. Believe me when I say that carrying a camera over your neck is going to be very tiring when you have to do any activity besides point and shoot. The new SLRs, especially FX ones, are as heavy as their predecessors.
Having said all that, a bad camera bag is as effective as holding your camera in a plastic shopping bag so choose carefully.
My tips are as follows:
1) Think about what activity you will or do use your camera most.
2) Determine if you are going to be active with your subject. What I mean is that if you have no intention of running or climbing around to take the shot then you might want to think of a bigger bag to fit more equipment. Small backpacks can be carried around very effectively but if you are doing a wedding shoot you might need a variety of tools.
3) Find out what lenses you do use 90% of the time. If you do sports photography you will be carrying around anything from 300-500mm lenses which are bulky and long. A bigger bag will be an absolute. If you travel a lot something smaller for your 17-50mm will be sufficient and maybe you can accommodate a prime lens as well. If you use though your 17-50 most of the time and buy a bag for the occasional 70-300mm lens or the macro lens that you rarely use outside the studio setting or for that occasional project, then you will be making your life more difficult.
4) To tripod or not to tripod? That is the question you should be asking yourself. A tripod improves your picture in almost any situation. I say almost because unless you are a celebrity or press photographer then you are running out of excuses. I know it is bulky and heavy but if you could go back to shoot that one picture again you would take that tripod with you. I have had plenty of trips in the beginning of my photo career where I did not take a tripod with me. I don't think I can count the times that I said "I wish I had a tripod for this one". The best shots are at dusk or early morning meaning long exposures and yes during those hours a tripod is your only workaround. If it is about the picture it is always recommended. Here is a video showing some easy ways to attach one to your backpack. I will write about tripods in another post.
One more thing: If you are starting off and don't have much gear to carry around then you can use something small like the Lowepro Slingshot 100
Generally I am a fan of Lowerpro bags and I personally use the Lowepro Fastpack 350 at the moment since it fits most of the gear I want to carry around and is very comfortable. It also has three things I really like, A) I can use it to carry a laptop if need be (I can easily fit a 17" laptop) B) The side pocket for getting quickly at my camera and C) Great padding for the back and enough straps to secure the backpack on you even when climbing.
Whatever your choice, have a clear picture in your mind before buying of what your needs are and how the bag can cover them. As your gear grows so will your bag size but keep realistic goals of how much you need with you. I hope some of this helps.