I have one. The first time I used it, it was all over the place. Unknown to me at the time, I had a vacuum leak which was causing the setup calibration off of carb 2, to be much lower vacuum than when I hooked up to any other carb.
After testing the calibration on each of the four ports it dawned on me something other than sync was causing the gauges fluid to go up high.
After checking every single hose on the bike from top to bottom and finding/fixing a hole, it worked like a champ.
I do recommend that you calibrate it on the low side (Fluid just rising up about an inch) at first and then get the carb sync close. Then re-calibrate it at a higher level (1/4 of the way up the scale) and do the fine syncing while blipping the throttle between adjustments.
Hope this helps.