How neat? How much money is it worth to know this?
There's only been one time in the last 7 years when it might have been worth it -- When we sailed from Costa Rica to Hawaii we burned 70 percent of the fuel in the first 1200 nm (of 4800) trying to find wind. We used the engine manufacturer's graphs of RPM vs fuel consumption to help estimate how to get of max distance / gallon, but it wasn't precise by any means. We were also forced to estimate the fuel remaining by measuring what we were pumping from each of the tanks -- which works fine in most circumstances, but in this situation, having a more accurate estimate of what remained in the tanks would have been valuable information. Winds on that route were light that year and as the hurricane/cyclone season was rapidly approaching we wanted to keep the boat moving down the rhumb line.
It it was a few hundred dollars, I might consider for the next round of capital improvements. If it's much more than that, we'll wait for the next time we have to motor to Hawaii.
The product I was looking at is made by Floscan http://www.floscan.com/html/blue/commercialmarine.php
I think the issue w/ diesel engines is which "pipe" do you put the sensor on? You really need two measurements: one of fuel flowing into the injection pump and one for the fuel flowing down the overflow line after the injectors. Then you need a small 'black box' to calculate the difference. I guess that's what makes it expensive.