I started with a diaper backpack from Babies R Us. It cost around $75 and I thought it would be perfect. That is until less than 2 months later, it started coming apart at the seams. I do think a backpack is better than a shoulder bag, especially since I had twins so i really needed both hands free!
After that, I was so bothered that I'd spent that much and gotten a product that just didn't hold up, I ended up just looking at regular backpacks. For just a fraction of the cost, I found several good quality backpacks that would satisfy my needs. (and lots of pockets, etc) I ended up going through 2 more backpacks over the next 4 years and those two didn't cost together what the diaper backpack cost by itself. I've heard people say they love the eddie bauer backpack diaper bag but to me, it's just not worth the extra money. Look for one that is not made of that slippery parachute like material (I'm drawing a blank at the name of it) because that material will just pull at the seams and cannot be repaired well. Really think about what you will be carrying and make sure there is room for everything plus, and enough pockets so you don't have to pull out a bunch of stuff to get to the rest that's on the bottom.
good luck!