In the end it all depends on what you want out of your pack and how you use it. The things I want out of my fishing pack, on top of carrying enough flies and fishing gear, are:
-ability to carry water and food for a long fishing day
-ability to carry a stuffed rain shell/layer
-effectively carry a net if wanted
-do all this as comfortably and efficiently as possible
After trying one of the smallish chest packs and learning that I hated the bulk on my chest, I opted for the Patagonia Stealth Sling pack
I have only used it twice, but am quite pleased with it so far. Last week I hiked and fished 3-4 miles and was thrilled with how comfortably the thing fit. When it is on your back, it is completely unobtrusive and easy to forget it is there. The small pocket on the shoulder strap conveniently holds little stuff i.e. split shot, floatant, chap stick, chamois, tobacco...or a small point and shoot camera. I found it nice to not have to dig around to grab my camera if I wanted a quick shot of scenery, or a fish in the net.
The main pack has the zip down fly "bench" with a ripple foam insert. There is a nice row of three small tippet spool-sized pockets inside that section and one larger divider pocket for slim things i.e leaders. I removed the foam insert and use that outside pocket to hold one large sized fly box, it takes up all of it. The foam insert would hold quite a few flies and leave space to use the pockets in that compartment. I opted for a big box and hanging my tippet on the outside.
The main pocket I put a second large box, a soft streamer wallet, a PB&J sandwich and could still stuff a rain shell in there. It's tight, but it fits. With it packed that tight I hang tippet spools, nipper, and forceps on the outside. Now normally I HATE having stuff dangling and clanging all over and catching line etc (which is one reason I don't like the chest pack), but on this pack it works so well because hanging tools on the attachment point puts them in easy reach when tying on flies, doing leader work, etc but as soon as you swing the pack onto your back everything is out of the way and can hang freely and not get in the way of anything.
The water bottle holder on the very bottom is one of the smartest yet simplest placements, very convenient and functional.
Last time I used it, I clipped a net on the ring and fished for a few hours. I works well and stays out of the way for the most part. It is obviously noticeable for the weight it adds, but when isn't a net swinging on your back noticeable?
For a big hike-in, long day I would still opt for a daypack to carry enough layers, food and water. But this pack carries just enough for a day of wade fishing and carries it really well. You can put the things you need/want accessible within easy reach. And while fishing, with the pack on your back, you honestly can forget it is there. It is comfortable and functional. That's all I can ask for.