Products don't often need to be perfect in order to be useful, but the Coin puts itself in a position where it has to be. Unfortunately, it isn't.
I love the idea of Coin. I usually carry three credit cards with me, and I have four more that I don't usually carry because I use them so infrequently (I like having a slim wallet). The Coin promised to let me always have all seven cards with me, all the time, by just carrying their one universal card. But this promise is only kept if it works every single time I go to swipe it. And it doesn't.
I understand how difficult it must be to get a card like this to work on every single card reader in the world: it's a daunting, monumental feat. But it is the crux of this product's success, and it simply does not work 100% of the time. The card is unrecognized, or actually rejected, by too many card readers. Really, one is too many. As soon as your Coin is declined a single time, you need to start carrying the real card with you as a backup for when the Coin doesn't work. And now you've gone backwards in your quest for a slimmer wallet.
One time is too many times to fail because you aren't always checking out at the Apple Store (one of the places my Coin was rejected), or buying clothes at a department store. When it fails then, it's an inconvenience if you have to go home to grab your real card and return to the store to make your purchase. But if you're at the drug store buying medicine for a sick child, or trying to pay for a meal at an expensive restaurant, encountering a reader that won't swipe your Coin is at best embarrassing, at worst dangerous.
This isn't to say that the Coin had to be perfect on every front. It only had to do the one thing it promised to do perfectly. All the other little annoyances could have been overlooked or ignored. The Coin is still a very impressive piece of technology; an exciting glimpse into a future that still requires physical cards (just fewer of them). Cashiers routinely ask about it, and are impressed by the technology (except for the handful who experienced it not working). But it isn't slimming my wallet, because I can't trust it to work 100% of the time.