Studio Acoustic Tiles/Panels for listening room?

One problem with using too much of this material (i.e. 2", 3", 4" thick) is that it is not linear through the audio range. It attenuates less and less as you go down in frequency. Therefore, if you cover the whole room, you will have an unbalanced sound. Now, if you could have an ideal room with professional quality absorbers (very thick) that responded linearly down to a very low frequency, you wouldn't have to worry about this as much. Keep in mind that this is all theory, I've never experimented with this. But, as a general rule, the thicker the absorber material, the lower the frequency it can absorb.

Alexc hits a good point. If you don't have absorptive material at your first reflection points, you are smearing your image pretty bad. This is not a big problem if you have your speakers on a long wall. I had mine set up on a 20 foot wall and my lateral imaging was good. When I went to the 14 foot wall in the same room, I had problems. Sound absorbers, drapes, etc at the first reflections points, really made a big difference in this case.

To find your first reflection points, hold a flashlight or laser pointer at the top of your speaker and shine it on the side walls while someone else holds a mirror flat against the wall. Adjust the flashlight and mirror until the reflection hits your listening position where your head would be. Now, mark the spot where the mirror is at. Do this for both speakers. Then do an experiment of listening to some very familiar music without any absorption and then again with some type of blankets, absorbers, drapes, etc. You should be amazed at the difference.