I've used both a homeade manometer, and the carbtune. Naturally, I prefer the carbtune, but the homeade version did work. You still need a vacuum gauge in order to use the homeade version, but not for the carbtune. It is a good tool to have for those that don't mind spending money on tools. I've made good use out of mine, mostly on other peoples bikes.
Oh, and I bought mine used, so it was an even better deal.