DISCLAIMER: These impressions of Miitopia are based only on the demo – not the full version of the game.
Miitopia is Nintendo’s attempt to make a full-fledged game for 3DS using only Mii characters. While the idea sounds good on paper, judging by the demo, Nintendo needs to go back to the drawing board with future Mii games like this.
Miitopia is a traditional JRPG with a battle system that is almost completely automated. You only have control of your Mii character in battle. The other members of your party are controlled by the game. As a result, you spend more time watching the battles than playing the battles yourself. The game plays itself to a large extent. As the player, you get to sit there and watch.

Miitopia puts a big emphasis on the Mii characters and their relationships with each other. As your characters bond with each other, it unlocks abilities in battle. However, the relationship aspect of the game is over-emphasized at the expense of gameplay, even interrupting battles as your characters level-up their relationships with each other.
The gameplay is almost brain-dead in how it holds your hand. Even the exploration is automated. There is no exploration outside of the map screen, which allows you to choose where you want your Miis to go next. Then your characters walk in a straight line through that area of the map. You have no control over them.
Miitopia is basically Nintendo’s Find Mii StreetPass game x100. However, Find Mii was designed as a StreetPass game. StreetPass games are designed to be played in very short bursts – not as an RPG that takes dozens of hours to beat. Plowing through 30-40 hours of Miitopia, and paying money for it to boot, would be mind-numbing.
The one positive thing about Miitopia is its writing. It’s very silly, but funny. Your Mii characters have amusing conversations with each other when they’re relaxing at an inn, or when they’re out in the world exploring. What there is of the overall story is equally silly and funny, but it’s not nearly enough to justify paying money and time to play.

VERDICT:
Judging from the demo, Miitopia is a game I could only recommend to a child as an introduction to the RPG genre, or to someone who has never played an RPG before and enjoys having Miis in their game. Other than that, Miitopia has little to offer outside of the novelty of playing a full-fledged RPG with your Mii, and the Miis of people you know in real life.
The demo has character and some funny moments, but the gameplay is almost completely flavorless and bland. I’m not sure where Nintendo saw the fun-factor in this kind of gameplay. If you like RPGs, and have been playing them for any length of time, I would recommend passing on Miitopia. There are plenty of better RPG options on the 3DS.