I've been using this bag for a month now and am starting to like it v much. I've been using a Patagonia sling bag for years and have also tried the Osprey. Both are much more conventionally designed. The folks at NIID seem to have redesigned the sling bag from first principles--it should almost come w/ a manual!
They've tucked in pockets all over the place, using a lot of space that goes wasted in conventional slings. I tend to carry a lot of junk around with me ("just in case," lol) and haven't come close to filling it up yet. They give you TWO water bottle holders. When my dog and I go walk in the woods, I can bring both a water bottle and an 18" Chuckit + ball. I put it in head down and cinch the handy little cord around the handle. No more lost chuckits! The bag works best on your back; when I need something, I slide it around to my chest and retrieve/replace the item. My wallet is safe and secure in the almost camouflaged pocket that rests closest to my body, which is brilliant. The main compartment is huge and thoughtfully appointed with a sturdy divider and both zippered and unzippered pockets. It's a joy.
In some ways the NIID seems over-engineered. The little removable "wing" pocket tends to kind of flap around, but it's the best place to stow keys. Annoyingly, the key clip is outside the zippered pocket; they seem to want you to tuck your keys into a thin, zipperless pocket in the wing, which would be fine if you only have 1-2 keys. But my keys are too bulky and need to go into the zippered pocket, rendering the key clip unusable. A 2nd key clip inside the pocket would be helpful. The wing attaches to the bag with one clip and a cool but awkward slide mechanism that to me feels fussy. A couple of times it's slid off its little track, leaving the wing hanging by just the clip. I feel like this could be more secure. The magnetic closure is well-built and works fine, but a regular and lighter plastic clip would work just as well and would lower the price and weight of the bag. The buckle that lets you adjust the length of the main strap has not so far been very stable. It has a tendency to slide up, making the strap as long as possible, so I often end up having to re-cinch the strap to where it's comfortable to me. It might help to rub candle wax into the webbing of the strap, to give it more friction and encourage the buckle to stay where I want it.
The NIID lacks two features that I love about the Patagonia bag: the 2 cinch straps on the outside of the bag. It's great for stowing a rolled-up shirt, clipping my hat to the bag so I don't inevitably leave it somewhere. Once I found a perfect 4-point shed antler; I strapped it to the outside of the bag and got it back to the car undamaged. I've already wished several times that the NIID had those outside cinch straps. The other thing that's missing is a small, tough strap for hanging the bag on a nail or coathook. These are pretty standard and I can only think they left it off for a more streamlined look. But hanging the bag by its wide main strap is needlessly awkward.
Overall, I really like the NIID. It's well-made, good looking, comfortable (to me, anyway). The design is definitely unorthodox and takes some getting used to, but once you do, it's convenient and easy to work with. It has a couple of features that seem overbuilt and under-thought, and it lacks a couple that I miss from other similar bags. But I'm very happy with it and suspect I'll have it for a long while. If I had ten stars, I'd give it 9.