I have had the S-AFC installed in my 91 for over a year now. I had it installed with my CTSC, as a way to fine tune the AFR. I originally wanted to go with the one that controlled the Vtech engagement, but APEXI(at the time) said they didn't support the application, and the guys at the shop I got mine installed at didn't feel comfortable trying to install it and have something go wrong. I know some people have installed them in their X's, and I don't know if some of those people have posted exactly which wires to tap into on the ECU yet.
It is very useful in fine tuning the AFR, especially if you have a good group of mods, and the good thing is that you can continually update the settings whenever new goodies are added to the engine, and it isn't very difficult to do so. You won't see any huge improvements with it unless your AFR was significantly off before, but you can at least be assured that everything is running at top effeciency with what you have.
Also with these units you get readings that include throttle position, RPM, manifold pressure, and a few others, but there is no way to turn the unit off, that I have seen. You can reset the unit to baseline, which would mean that you will be running whatever maps your ECU has in it (effectively bypassing the unit).
As for tuning it, you need to do that on a dyno, with a gas analyzer, or some tool that can tell you if you are running lean or rich, but if you have access to one of those, tuning it yourself is pretty easy.
-Stuart