I visited Mangaia Island in 2012 and spend few nights are this place. Well, the only positive thing is the owner (a sweedish guy), very kind and helpful. But the problem with this place is that there is only one customer going every 2 months or so (Mangaia is not a typical touristic place and for instance, during my week visit, i was the only tourist on the complete island !) so at the end, the place is almost empty 95% of the time. The owner also don't stay in Mangaia during this time and he's flying back to Rarotonga where his family is living. This means 95% of the time, the place is abandonated. It's a pity because there is a lot of potential for this place and everything has been made by the owner, so when you speak to him, you feel he's very proud of his work, but at the same time, he realise he can not maintain it with only 1 customer visiting every two months.
Since the accomodations on Mangaia are very limited, this one has the advantage to be cheap and well, it's still possible to get a decent sleep and correct food (prepared by a local lady). But better to be prepared and understand this is not the place where you can find any of luxury or comfort.
On the other side, Mangaia island is a very interesting island, totally different from others Cook island (almost no beaches, no tourism industry etc ..), only recomended for aventurous or backpackers travellers (don't go there for a romantic or family getaway ...)