Sorry man, I linked you to the wrong page... Yeah, your link above is what I meant to link to. And yes, it's just an A/F ratio meter, and not a very accurate one from the way it looks. It just tells you whether you're rich or lean, but doesn't show a good scale of HOW rich or HOW lean. I'd go with the autometer if I were you. The reason that MSD device costs so much is because it comes with a heated O2 sensor, and has to have a heating control circuit. Since you already have an O2 sensor on your car, you can buy the Autometer product, and get a better product for less money... Just my opinion...
You can adjust your fuel pressure, and see a difference in how rich your car is at wide open throttle with the Autometer unit. At idle and regular driving speeds, you'll see the Autometer jumping around because the ECU is trying to adjust for a stoichiometric mixture. That MSD would be total garbage if you ask me... It only has two indicator states, rich and lean...