Fishpond Waterdance Guide Pack for about 5 years now, and it is by far the best on-stream storage system I have ever owned. It was my first step from a fly vest to a pack, and though it looks much smaller, it's storage capacity was easily double that of most vests.
I wanted a pack that could hold everything I need on the stream so I could just grab my waders, boots, rod, and pack and hit the road for a fishing trip. I store 3 reels, 4 fly boxes, split shot, frogs fanny, leaders, tippet, etc. in this bag. It has two water bottle sleeves on either side that I usually use for indicators and little plastic cups of flies. The front pocket unzips to look like the inside of a fly box. This is where I store streamers, san jauns, and flies I don't want to return to the correct box when I'm on the stream and in a hurry. The middle compartment has a hook for you car keys that I always use. The middle compartment is where I store my reels, leaders, and other miscellaneous items. The back compartment is the big one where I store my fly boxes in. It is constructed of primarily ripstop nylon, so flies don't tend to get permanently lodged in the pack.
The pack can be worn as a chest pack or a lumbar pack. I tend to wear it off the front and to the left to leave room for casting. I've found that as full as I keep the pack stocked, it can get in the way if I wear it on my chest. If you don't remember to zip a compartment back up, when you lean to land a fish, you may dump a lot of its contents in the river. Thats user error though, not poor product design. When this pack is empty, it is super light and compact. It almost functions like an accordion, allowing you to easily store a lot or just a little gear. If you need a new pack or vest, I highly recommend the Fishpond guide. I also have an Orvis sling pack that is cool, but it doesn't compare to the the Guide.