I have them on my twin CAT 3208's. Have found them very accurate, +/- a couple of %. They are set up on their own N2K network and connected to a DSM150. They are the only N2k devices on the boat.
The bit I like best is fuel
burn in real time. This allows me to plan my trip based on fuel that i want to burn, rather than time to get there. I can adjust in real time due to wind
and currents etc. It also allows me to show fuel used for each trip, which helps with getting my mates to chip in for their share!
I have just purchased a new (2 me) vessel that has Floscans. From my research
these are meant to be more accurate, but I will not be able to comment on that for about another month or so, when I pick it up and do some cruising.