I have 4 of these in an array on the back wall of my home studio. The room is everything you don't want a studio to be - small, almost square, with low ceilings and all 4 walls are masonry. I had already done the usual sound treatment with absorbers behind the monitors and on the side walls in a live-end / dead-end design but was still getting a nasty ringing sound in the room. I decided that I didn't want more damping and tried the Hofa Diffusor to try to tame the acoustics of the live-end. It seems to work quite well. The nasty ringing has gone and, though I think there has been some high frequency attenuation, the room sounds much better for performing.
The diffusors are really quite elegant and a number of people have commented on how nice they look. They also don't take up much space.
The construction quality is not that good. The panels are not exactly square and there are gaps between the in-fills and the side pieces in places. They came with no fittings! I couldn't understand why at first until I discovered that Hofa want you to buy a frame to mount the panels in - which cost as much as the panels themselves. The panels are made of extremely thin birch ply so there is no way you could screw a bracket to them. I resolved the problem by screwing wooden mounting blocks into the wall and just hanging the panels off them. It took a very long time to do this and get all 4 panels lined up with each other, but it worked very well. Beware if you don't have decent DIY skills.
Update, Aug 2019. I bought 2 more panels hoping it would improve the sense of space even more. What a learning experience! The extra 2 panels completely altered the sound of all my instruments, weakening the bass response and muting the highs. I spent hours trying the panels in different positions on the back wall and on the side walls and the results were the same everywhere. I discovered that the instruments sounded best with 4 panels on the back wall and playing as far away from them as I could get. So, whilst I don't really understand what is happening, it appears that there is limit to the number of panels you can use in a small room.