I have had the red cube installed from the beginning, 10 years ago, and I kind of like it. It certainly is not necessary when you have direct reading sight gauges, but it gives me more accuracy than a sight gauge, which I enjoy. I never run so low where the accuracy of the last gallon or so becomes important, but its nice to know its there. Installation was quite easy and it reads out right on my Skyview panel.
You do need to know that with a Rotax with a bypass return line you must do one of two things:
1 - Use two red cubes, one in the main fuel line and one in the return line so the actual fuel flow to the carbs can be determined-this is expensive and defies the KISS principle.
2 - I used just one cube in the main fuel line and carefully calibrated the "K" factor to eliminate the return flow, which is very small anyway-on the order of 1/2 gph if you use the Rotax recommended return orifice size. This calibration takes several trial-and-error runs to get it right.