I just got finished creating a recording studio in my basement and put up a small vocal booth 4' x 4' with an exterior 9-light door. I was at the end of my budget, so I went on the cheap, but saw this as a must-have. The construction was merely framing, w/ 3/4" subflooring as the interior walls and drywall for the exterior. I used an acoustic adhesive (much better than the sheeting you can buy, IMHO, and easier to work with) to contain the sound between the booth and the larger room. The interior of the booth was deadened with my last panel of 2" fiberboard insulation cut & friction-fitted between the framing, though it may require some acoustic shaping after that (which is why I save every piece of the dense, egg-crate foam that computer equipment, etc comes packaged with). The foamboard is pink, but I'm covering it with fabric soon to keep looking as nice as the rest of the new construction. I also drilled a 1-3/4" hole in the bottom of the wall and lined it with foam pipe insulation. Now I can run instrument cables, headphone cable and mic cables into the room with little-to-no loss of isolation.
Anyway, to your point, I don't really have before and after samples of the sound quality, but I'll give you the reason it was a must-have for me. I do computer-based recording and have some high-quality condenser and cardiod mics. They constantly picked-up the computer fan sounds, etc. I also had a few great takes ruined when I moved in the chair and a loud squeak came over the mics. That has yet to be duplicated with the new iso-booth. Plus, it wasn't spending tons o'money to build it. It took about 2 hours to construct it (not including drywall finishing and painting).
I hope that adds something worthwhile to this thread.
Craig