The effectiveness of these panels varies depends on the size, shape, of your room, your speaker positioning, your listening position etc etc. If you are really interested google bass traps and sound diffusers... they are basically two types of acoustic panels...
the improvements, well, are much more apparent than say my upgrade from my Onkyo amp to a NAD M3, the later cost 3 grand new, so the 100 bucks I spend on these entry level foam panels beats more than "2k" spend on amps. of course I didn't spend that much on the amp but you get the idea. placement of the panels are important as well and you don't want to over treat your room as well
My speaker's are omni-design and they don't really benefit from these panels as much as conventional front firing speakers, I can just imagine how big an improvement it would be if I still have my Lsi9s in my listening room.