Bare in mind it's not "real-time" it's a reading every 2 or 3 seconds, yes it's close to real time but if you keep thinking that it's genuine real time the moments where you see the figures go from 10lph to 3lph then skyrocket to 25lph when you barely move your foot you will go batshit trying to figure out how to change your driving habits.
They are a good tool, I don't believe they promote economical driving the way their website and manual says they do because I don't believe anyone is that finite with their throttle control but there is no doubt it's a good guide and the averages over the entire tank are as accurate as the person who set up the gauge.
If nothing else the accurate speedo reading give the missus something to comment on if you are a lead foot and she likes telling you off for going over the limit. The water temp isn't bad to have under some conditions and for trips like you are about to do across the country the "fuel to empty" (once again when set up right) is invaluable especially if you have your favourite stops across the Nullarbor like I do.