I've known about the e-Reader for a long time (and saw it advertised in things like Animal Crossing), and I got Pokémon cards as a kid that had dot codes on the sides but didn't have a way to use them. But I got an e-Reader very recently on a whim, and it's very cool! I got Pinball and Excitebike, the Air Hockey card and some Animal Crossing cards, in addition to the Pokémon cards I already had.

The NES games take 9 or 10 scans, which sounds like a hassle, but scanning each card is easy and doesn't really seem to require much precision when the card is in good condition, like swiping a credit card. I've actually had a harder time scanning QR codes sometimes. And you can save the NES game to the e-Reader to play later without having to scan the cards again, until you want to save a different game. It's unfortunate that only the smallest NES games are supported, but if I had to do like 20 scans and the payoff is Super Mario Bros 1, the hassle might start to outweigh the novelty a bit.

The cards with original software are cool too; the Animal Crossing villager cards have a little letter from the villager with a chiptune version of some of the game music. The Pokémon cards I have encode the minigame for Fire Hoops which is only really fun in a Flappy Bird kind of way, so it's not really for me. I like Air Hockey though, and it seems like a pretty cheap card. The Pokémon cards also let you look at information about the Pokémon, which is neat. It does make me wonder how much of the stuff I'm experiencing is actually stored on the cards or not; I saw from reverse engineering efforts that there are a lot of Pokémon sprites and other assets and code available for cards to use, stored in the e-Reader itself.

I'm interested in checking out Mario Party-e someday; I love the idea, though I don't know if the execution was good? It'd probably be tricky to gather people together to play it too.

Most of the stuff you can play is very simple and maybe not elaborate enough to be that fun for very long, but I think a lot of the point of the e-Reader overall is that it provided a way for very simple and small experiences to exist on the Game Boy Advance in the first place. You would not make cartridges out of these games (except in the case of the Classic NES Series, which is also weird to have exist), but you can print some paper cards for cheap and get the games and toys out for people to play that way. It's similar to how the cheapness of CDs allowed for things like demo discs and free discs in magazines to exist. Nowadays with Internet connectivity, tiny experiences can just be free downloads, and if someone did a similar gimmick it would probably just unlock experiences within some app rather than actually containing them. Actually, now that I think about it, the Scott the Woz trading cards that contained a QR code to a timed exclusive video is quite a bit like e-Reader...

Anyone else try out the e-Reader, either whether when it was current or long afterwards?