This is interesting, because I did the same, with some efforts to use the same fabric as the curtains' and matching their color with the walls. The effect is acceptable to me, but a carpet on top of the panels would have a better WAF (at least with mine). This should work similarly to the panels alone at medium-high frequencies, maybe a bit less absorbing at medium-low frequencies and, if you "frame" the panels (with a wooden perimeter with the carpet fixed to it), should work a bit more at low frequencies.
These panels are intended to reduce reflections from the back wall towards the listening area mainly for medium-low frequencies up, while tube traps in the corners should better fight the room resonance modes, which are not an enormous problem in my room.
The right side of the room being a full wall window I put only a light curtain on it, with a high chair in the way of the "first reflection" to the listening zone. Using a heavier curtain would improve reflections control, but I don't like a too dark scenario.
Adding a diffuser on the ceiling would also make good, I am thinking of what type not to look too weird. A "skyscraper" type would work grat, but I have to improve its WAF.....
Anyway I am reasonably happy of the situation as it is now, so I have little driver to make further changes.
Lately I preferred to spend efforts in my taping rig, PC interface with the the HiFi / HT system, large storage HD, software for editing, etc., for which I found great guidance on this website, which is covering most of the sound recording / storing / listening areas I am interested in.
