I bought the Wayks Combo and in theory its perfect, but in practice you will end up in so many situations where the Sling is just too narrow to be useful. The Mini is also a tad too small for my essentials (see below). I appreciate the work they have put into this product though, so please see this review as encouraging rather than destructive.
If the Sling was about 10cm wider it would cover most situations with ease, both as a backpack and sling bag. But right now, it does neither.
For example: If I bring a large water bottle, I have to put it upright and bring the sling as a backpack. Same with a 750 ml wine bottle. Same with my (admittedly large) foldable umbrella. This constant switching between backpack and sling bag-mode is made worse by the fact that there are six steps in changing the straps from backpack to sling: You have to remove backpack straps at 4 places, and add sling strap in 2 places - and you can't keep both on at the same time.
So you might ask: Why not just always use it as a backpack? Well, even then, it just barely fits things you'll need for a normal day.
Work? Can't fit macbook pro, water bottle, food box, and thermos.
Beach? Can't fit towel, large water bottle, and a jacket (not to mention snack food box and blanket to sit on).
Meeting friends outside on a spring evening? Can't fit a wine bottle, sweater, and either one of a water bottle or a 40cm umbrella.
Biking? Don't expect to put much more than a foldable(!) bike helmet.
Shoes? If you wear heeled shoes or flipflop, a pair of back-up sneakers probably won't fit.
If I could do this purchasing decision again, I would probably not buy neither one of the Sling or Mini unfortunately. I would buy a slightly larger sling bag than Sling Mini with as many pockets as possible, and wear it all the time. Then I would have a complementary normal-sized backpack or an XL sling bag, to put bigger stuff in while still wearing my small bum bag across my neck, so I still can access my essentials easily from it.
The reason I bought Wayks Combo was to not have to do this, but rather always just have one bag with me for all types of day excursions, from office to beach, where I can easily access my important stuff (in the Mini) but also expand and contract the bag for spontaneous situations like shopping on the way home.
The Mini can fit a lot if you cram it, so to be fair I will list what I have inside and what I would wish to have space for - maybe the 0.5 liter space will be enough place for you:
My Mini fits keys, small earphone case, earplugs, phone charger, cable, mini portable bike lights, one pen, sunglasses, chewing gum, condoms and a bike saddle cover crushed into the tiniest imaginable ball possible. Note that everything is extremely crammed in this way, which hinders accessibility. I only fit these by playing Tetris in the Mini.
I would wish for additional room for: My private and work phone, my passport, my powerbank, cash, tissues, hand desinfectant, a small lotion.
Nice to haves (but maybe not as essential for me) would also be a very small notepad, a pencil with a small packet of lead, bandaid, painkillers and sanitary pad/tampons (backup for my gf and female friends).
Now, I know what you are thinking - I could keep all of these in the Sling and just attach the Mini to it. But the Sling would then have a lot of empty space just in order to have some additional small things inside that should just fit in the Mini instead. And seeing as the size of the Sling doesn't really allow for bigger things, I might as well just put all of it in another slightly larger sling bag instead (like a 1L one). So the Sling is unfortunately quite useless for me.
One very positive note on the Mini is how the square structure keeps your stuff in place (you can stack them nicely), in contrast to many small bum bags that are so soft it feels like searching for your essentials in a plastic bag. If the Mini would have a slightly larger size to fit the things I mentioned, it would be perfect in my opinion. Or even better if it was possible to zip two minis together, in which case different needs would be met in a smooth and modular way. So the Mini is not far from being great as it is. But the size of the Sling itself unfortunately has a long way to go to in order to be practical, as some others here have mentioned.
Either way, the build quality of both products are excellent. If you are a person who usually only brings very little/small things with you, I can highly recommend the Mini at least. And if you are thinking of buying the Sling, just think twice about whether the backpack dimensions are good enough for you. Compare with Wireds backpack size guide for example. Perhaps the Sling will fit your needs, after all.
Anyway, thanks to Leonie and Fabian for the work they put into this. I'm sure it meets the needs of other customers. And if there will ever be a wider sling and/or double mini, I'll be th...