I have also built one myself and I enjoy using it. I never practice outside. I enjoy practicing any time I want and any volume I want. Also, I noticed that (maybe because of the damped practice) my projection has really improved outside.
The deadness of the sound depends on a. the size of the box (the smaller the more dead), and b. the amount of damping material I put on the inner walls. Mine is roughly 6 by 4 feet outside (or even a bit smaller). It was around 4 by 3.5 first, but that was way too small. My teacher has one sized 7 by 6 feet (see www.studiobox.de
) and he teaches in there, what means: two guys, a computer, speakers, 2-4 saxes, note stand and everything. Works fine and also sounds good.
Playing against a wall sounds about the same inside and outside. The 'industrial strength' boxes surely work better in terms of sound, not in terms of sound-proofness, mainly because of size. I am pretty sure that the sound of the self-build vs. the bought would not differ much, given you have them sized equally. The effort and cost of building a booth is not neglectible, though. If you build same quality and flexibility (dismountable) like e.g. the studiobox, I don't think you could compete.