I have a ministack, and I can confirm that the unit does feel somewhat 'cheap' especially the clip on top part.
The fan can be loud but does not run constantly, it is temperature controlled, when the temperature gets too high, it kicks on. However, I found that if I put the ministack under my mini, then the fan stayed on 24/7. Also the fan can get confused. I'm not sure how or why but sometimes it will start up and not turn off until you power off and power up the ministack again, then it behaves normally.
Overall despite the things I've noted here, I'm satisfied with the ministack although the build quality could be a bit better with the top piece.
They do have an updated version of the ministack, I'm not sure if shares the same issues I've addressed here or not.
Update for those who stumble here. My ministack has since died....