they provide a good amount of scattering. That's to say, sound that would normally hit it and bounce in one direction is dispersed evenly along the arc. In terms of what it did subjectively for my practice/live room, was that reflections were a lot more even everywhere, and finding room mic locations is a breeze. Because sound is dispersed, no reflections are as strong, and they seem to be coming from nearly everywhere.
what they do not do well is time based diffusion. i'm not really sure how to tell you how this "sounds" because I don't have a lot of experience with qrds, prds, etc... I do know though, that poly "diffusers" are typically not the prescription for back walls of control rooms, because they lack this temporal (time) element.
there is some amount of bass trapping going on here. again, that rockwool fill and the sealed cavity are what's doing the work here. Andre (avare) was instrumental in helping me design these, so he's the one to ask in terms of calculating their performance. What I can tell you is, this is primarily a practice space. With 5 vocal mics set to be heard above a 5 piece rock band (read loud) before the polys were installed, there was a lot of notching going on on the primitive 7 band eq on our Mackie 808s. After the installation, it is running flat, and is crystal clear. It's a bit of a spoiler actually. We've yet to encounter a club that can give us a fraction of quality stage sound compared to our practice space.
Cost? Hmmmm.... the plywood face was "free" as it was salvaged from previous "poor" treatment. I'd imagine a 4x8' sheet of 1/8" luan ply would run you $10 (dependent on grade)? So there's $2.50 a trap. Plus the frame...another $5? You've got an insane insulation deal so what ever 1 sheet of that runs you... some caulk, some screws, some finish washers, sandpaper, varnish, sealer.... Dunno... maybe $15-$20 a trap? Keep in mind the need for cabinet clamps... those ain't cheap.
And time. While you may be able to build these with your eyes closed, the bend time for the face is, or at least should be long. Take your time, and little by little, or else they will crack. If you want to build more than one trap at a time, your going to need 4 more clamps
In terms of breaking parallel in a live room, and a bit of sound spread without having to get too mathematical or complex, I think these things are great, and I would whole heartedly suggest them to anyone looking for the same!