I'll agree with Rich on some points and expand a bit if I may. I 100% agree that a mix of absorbtion and diffusion is in order in almost all rooms. Too much absorbtion can in fact make things dead and lifeless.
What we need to understand is that there is more at play here rather than just frequency response, impulse response, and killing early reflections. Every room for a specific application has a target decay time. Absorbtion will help you reach that. This is particularly critical in the bottom end of the spectrum. Once the target is reached (and with the absorbtion in the proper places) then diffusion becomes the finishing touch.
On the front wall with a normal speaker, I much prefer absorbtion. With a dipole, it's a slightly different story but that's for another discussion. The reasoning on the front wall is to eliminate smearing of the soundstage coupled with providing SBIR control. While diffusion makes the soundstage 'bigger', IMO it also causes it to lose focus. George (Zybar) tried my recommendation of moving his diffusion from the front to the rear and I think left it that way finding a more 'right sized' and focused image)
Rear wall of the room is a tricky thing. IF you're sitting far enough away, diffusion can work very well. That said, sometimes you'll have a huge frequency related null off the back wall down in the bass where only absorbtion can do the job. In that case, it's a trade off. Sometimes absorbtion in the center and diffusion on either side works - each room is different.
As for side wall reflections, that's a matter of taste. I personally want something that will deal with the entire vocal range for the reflections. That means absorbtion as even our diffusor only goes down in the 500Hz range (pure physics - they'd be just silly huge to go to 200Hz). Now, on the back half of the side wall is a different story. That's somewhere the is normally overlooked and untreated but can add a ton of life and spaciousness to the sound.
These are basically a derivation on a 7 prime. They're designed to work best in groups with a bit of spacing between them to take advantage of the wall as another 'well' and also to allow the additional reflection and scattering off the sides of the units as they're exposed.
You can orient them as you wish depending on situation. We're actually going to be doing some testing of a diamond arrangement with each one rotated 90 degrees to the previous and iwth about 4-6" between them. Not sure when that will be done for official purposes. What I can tell you is that we had a few customers who preordered and all of them have given us nothing but
so far. One person did the diamond thing on his rear wall and on the ceiling and absolutely loves the results.
Now if Glenn will just send me a few to play with....