Yes they work well, Your system, i applied right, will sound noticeably quiter. However your system temperature will increase by around 1-2 Degrees C.
One pack is ment to fit a Midi-ATX case, not a full Tower case, But it still works very well if you just leave some off at the top of the case.
More info can be found here. XtReMoDs - Extreme Alterations Akasa Pax.Mate Acoustic Absorption Mats Review