These make an actual difference, more than diffusers (which are barely noticeable) and more than even bass traps.
I didn't know how bad reverb can hurt music until I tried these. They really help with imaging as well.
Just don't go overboard with these , things can get scary really fast (yea, scary!). You do not want to turn your room into an anechoic chamber trust me! I only ended up using 2/4 for this exact reason.
Also keep in mind that their color is more gray and not white.