I have been wanting to get one of those. autometer has a new one and when it was put on display at a local shop they didnt have any in stock and it would take a few weeks to get one.
"perfect combustion" will occur at 14.7:1 which is 14.7 LBS of air to 1 LB of fuel (stoichiometric ratio). you want to shoot for probably about 12.5 to 14.5:1 so you dont get too lean. keep in mind its a guage, so it will tell you where you stand and where you should be.
you will have to install an oxygen sensor in your header collector or the closest possible point w/ manifolds..
ive never really talked to anyone that has used one, and K&N and i think edelbrock both offer one.. but if you know the concept, it would be awesome for jetting and setting mixture screws, etc.
theoretically the same effect can be achieved by just hooking a volt meter to the oxygen sensor and reading the output voltages but then you need a chart. I happen to have a chart from when I was in school and i'll scan it if anyone wants it (shows O2 sensor output in relation to air:fuel ratio)
Mike
PS- let me know how it goes!!! im really interested