I had no clue this was a thing until I saw posts about it on an online forum when it released. On a whim, I decided to check it out and added a bunch of people from there, and then those people's mutuals ingame. In the end I added like a few hundred people - but only a handful were still playing after a week or two, sadly.
Personally, I had a lot of fun answering the questions. I like the introspection and thinking about how to answer them both in an honest and a funny way. I suppose the fact that almost all people in my friendlist were strangers helped with giving honest answers...
And I liked seeing answers and pictures from other people. Some of those people also got really creative with the MiiFoto maker.
As time went on, some of those people who stuck with this app eventually became my friends. We exchanged usernames for a messenger app and I still talk to these people every single day.

It had a lot of cute features like room customization and tons of neat outfits. It's actually a pity that these outfits will probably never see the light of the day again, because a lot of them were very intricate and probably took quite some effort to make. Some of them would've been cool as costumes for Smash, for example.
The monetization was kind of weird... You could buy coins that could be used in the clothes store or to play Candy / Miitomo Drop (I think it had different names depending on if you set to British English or American English. This also changed the way the miis talked by a lot, by the way): a pachinko-like game where you drop Miis onto platforms to win prizes. Shoutout to Youtube User Aki-Games who made tutorials for all the different drops and probably saved me countless of tickets. Anyway, you had basically no good reason to spend coins ever. On one hand it's a good thing, but on the other it probably meant that the app made no money, which is probably one of the reasons they shut it down after just a few years.
In the end, it wasn't anything super exciting, but it will miss it dearly. ;_;