I own the Orvis slingpack. Its convienent, and its not as lame as a fanny pack. Its pretty much the right size to carry anything you'd need if you're not obsessed with carrying 20 variations of every possible insect in the eastern hemisphere.
It wears the weight well, and with the use of the waist belt, its almost not there. I tried my vest again this year after using the sling for a year, and it was unbearable.
On the other hand, I've simplified further, and found fishing more pleasurable with a box of flies in a pocket, and a necklace of goodies.