Ive a couple Qs here myself, please. and maybe an answer as well for the OP . or at least an alternative choice
First How low a ceiling does one have to have for these things to be of consequence? Or is there some specified distance from the top of the speaker to the ceiling in order for these gizmos to be effective?
Next Why would actual diffuser panels or squares be better and how does one decide how much and where to place it/them?
When I repainted and remodeled the living room for a theater, I wrongly listened to others and had the ceiling painted a similar but softer white. Bad move. The eye catches enough of it to be distracting as the screen is wall mounted, so it needs to be much darker.
I vaguely recall a wall paper maker that has a fairly good selection of fabrics and colors that act as diffusion and very slight absorption. Materials and are assigned the terms Acoustical room treatment fabrics.
The idea of diffusion on the ceiling and changing the color as well seem a good enough reason to put the difuse wall paper up there in my circumstance. As it wont catch the eye like individual panels will, and should prove quite complimentary without over damping a room.
It can be had in tiles or rolls depending on who you get what from. In one case I noted 12 tiles sell for $35.
http://www.mioculture.com/store/pc/viewCategories.asp?idCategory=2
www.noiseout.com
http://designerwallcoverings.com/WallpaperStore/index.php?main_page=index&cPath=590
others are out there for sure just keep Googling.