I have two pair. One pair is in the front corners of the room and a second pair is spread out behind the listening chair. They work very well in getting the room under control.
The construction of the Holographs is flimsy so if they are knocked over you'll need a tube of Gorilla Glue handy in order to put Humpty Dumpty back together again.
In addition to the Holographs, I have tube traps behind the listening position and ten Synergistic Research High Frequency Transducers (HFT's) placed around the room as directed by SR.
The HFT's were just as effective as the Holographs in dampening unwanted feedback from the room and in taming the smearing of the sound. Considering how effective the Holographs were, that's really a good recommendation for the HFT's.
The bottom line is ... this combination of room treatments has really had a very positive effect on the sound. Everything is locked into place. The system has a very believable, sometimes uncannily real, three dimensional presentation across a wide and very deep sound stage.
There is no way I would want to give up any of the above mentioned room treatments, the Holographs included. They all have/had their own particular positive influences on the overall musical enjoyment that is derived from the system at this point. Its important to realize that the room itself is just like another component in the system and should be treated as such.