I am buying a boost, A/F, and Fuel Pressure for my Mazdaspeed.
The Autometer A/F is pretty much not good for anything, but I am getting the Defi D-Gauge A/F that has actual numbers on it and is said to be pretty darn accurate. I will eventually get a wideband for tuning with a management system, but the A/F gauge is a much cheaper alternative in the mean time. Greddy makes a good functional A/F gauge as well, but it's not in Imperial units.