I want to give you an idea of the textures and construction of the wallet. To do so, I have to explain to you that the Slimmy is constructed from two pieces of material. One of the pieces of material is a thin sheet of leather. On top of this is laid a thin piece of textured fabric, like nylon, and when you scrape your fingernail across it, it makes a zipping sound. They appear to be glued together and the leather, which is slightly larger than the liner, is folded over and sewed along its edges. Now that the fabrics are stuck together, they are folded so that the liner is on the inside, and then folded again perpendicular to the first fold. The wallet is then sewed along the edge where the two leather creases are next to each other. This means that there are 4 flaps. The first has leather/liner, then liner/leather, then leather/liner then liner/leather. This creates three pockets, two of which have liner on both sides and one of which has leather on both sides. In the leather pocket, they recommend placing your credit cards and in the liner pockets they recommend placing receipts and cash.
The leather "sticks" more to plastic items, whereas the liner is more appropriate to holding in the paper items.
I bought this specifically because I wanted a wallet alternative. I wanted something that would both be slim, and yet be cleverly functional. The fact that it has three pockets is really great. Separating out your cash, cards, and receipts is perfect. Even "normal" wallets provide almost no acceptable way of storing receipts. Using the Slimmy will also teach you to limit the number of cards you carry. I carry four: a debit card, a credit card, health insurance card, and driver's license.
I also looked a lot at the All-Ett. The main difference here is that the All-Ett gains its slimness from the fact that you have two rows of cards. This means that instead of your wallet being fat, it's twice as high. It also introduces an extra step in removing cards, since it's folded over. I sort of view that as cheating. It's like saying that your cell phone is thinner, even though it's twice as wide and harder to open.
It really all boils down to a few things. Slimmy puts a limit on how many cards I can carry, which is a good thing if you're looking for a thin wallet, right? 20 cards in the All-Ett will still make it thicker than 5 cards in the Slimmy. Slimmy also provides a sensible way of dealing with the cash/card/receipt problem, providing the perfect number of slots, and providing the perfect texture for each. Slimmy makes it easy for me to obtain items from it; since each item is exposed, there's no need for me to unfold anything.