Miitomo is as good as the amount you care about your online friends and what they think about things. There's not much in it otherwise besides some light Mii customization and the only mini-game included is pretty terrible. It's more of a social app than a game, but I enjoy using it almost daily because, like I said opening this review, it's only entertaining and relevant if you care about that kind of stuff, and I do!
The 'core loop' of Miitomo, if you can call it that, is quite simple, you thumb through your friends and they answer questions. You can like the answers, you can comment on the answers, and then you answer questions yourself. The questions are a bit dumb, like asking you what you ate last night, and sometime they feel a bit too personal, like asking you about things to regret, everyone has a different level of openness about themselves on the internet, and your millage may vary.
By answering/commenting questions and from getting daily login bonuses, you get coins - and experience, but it's so irrelevant I won't even get into it - that you use to buy clothes or attempts at Miitomo Drop. You can also buy coins with real money, but so far I never was tempted to do so. Playing dress-up with your Mii is fun and I like how you can wear anything regardless of gender. There's enough selection with the clothing and it's goofy fun. You also have a graph of sort that you choose yourself to express what you think your personality is like and you can select a voice for your Mii, which is pretty good. The text-to-speech in Miitomo is silly, but it's a big part of it.
There are also Miifotos, allowing you to take pictures and add your Mii on top of them and add words, which then can be shared in-app or on social media. Miitomo drop is another way you can get clothing items, but it's terrible, it's like a pachinko game, but it feels unfair compared to regular gambling, as such, I only do it in cases of absolute necessity - i.e. I really want that item.
Miitomo is super neat, but it's not a game, it'a social layer on top of your existing network of friends that adds reasons for your and your pal to discuss about your favorite band or what kind of pizza you enjoy. Maybe it's also market research for Nintendo, but eh! I think I'll stick with this for a while.