g'day mate,
I had one a while ago with a pretty worked ca18 with aftermarket management and used the innovate LC-1 kit. It was hands down the most valuable and usefull tuning tool I ever used. Especially with an injection setup I could never go without one again, without one everything is simply guess work. The innovate kits are pretty much the best too as they really are the most cost effective for what they do, and they use the newer type/better (and cheaper to replace) bosch O2 sensor.
A carby engine might not be as usefull as once you have everything setup they still don't really offer the infinate fuel control an efi setup does. I would expect it would just show a heavily rich condition with carby's under most power situations. I would think for setting up idle and off idle mixtures and balancing carbs it would be very useull though. It would probably be usefull to have the gauge on the dash as a warning should anything go wrong with a jet or carby as you would see the result as an abnormal fuel ratio hopefully before anything got damaged.
But certainly EFI is where it is truly amazing as a tool because the mixture through the whole load range is controlled perfectly, not just a whole bunch a fuel crudely shoved into the carby throat but a finely controlled metered dose. Most aftermarket computers also interface with the kits too which allow another complex range of fuel control scenarios that you couldn't utilise with a carby engine like closed loop mixture controls or self learning functions.
Further to that I have an LC-1 kit with gauge that I was going to put on the datto I just sold prior to selling it and getting my mk1 racecar (best move I ever made). It's still in the box and would be happy to shift it for less than you'll find them on ebay. I was going to use it but like I said for a carby engine I don't think it would be as usefull, Not that I wouldn't like to use it but the car isn't quite complete yet and there are more usefull things I could fit before this. Let me know if your keen or have any other specific questions about their use.
Cheers,
Llewellyn