So far I've watched eight episodes of the series, and I'm enjoying it. It is a major-league fan service vehicle, as others have said, but I disagree with them when they say that the series plot is not "big" enough for thirteen episodes.
I may not be very discriminating; or perhaps stories like Popotan just appeal to me more than most people.
The concept for the series comes from a hentai computer game, in which the object is to have sex with the four girls living in a magical disappearing house. But the only thing left from that concept is the house, the character designs, and the fan service; the rest has been eliminated.
What's left is a trio of sisters and their robotic maid, in a magical house with a small retail shop attached from which they sell Christmas-themed merchandise. Every so often, the house disintegrates, travels to a new location, and reappears; and the girls go out and talk to dandelions in search of...something. (In episode five we learn that the "something" is an apparently young woman who travels much the same way they do, minus the house.)
Episode seven hints that there is a lot more to their situation than they know. The reviews of others say that the last four episodes are great, but I won't know that until tomorrow at the earliest, since I'm only letting myself watch one DVD per day.
Further bulletins as events warrant.