I've had the JPI 760 for several years now and like it a bunch. The JPI unit "talks" to all of the major GPS units, so in addition to just tracking how much fuel you are burning, if you have a flight plan entered in your GPS it will provide a ton of other information.
Most important, the unit will tell you what your reserve (at current ground speed, fuel burn rate and flight plan route) will be at your destination, and you can set a predetermined minimum fuel level. Mine is 20 gallons, or over an hour of flight time at normal cruise. Anytime the predicted reserve falls below that level the JPI flashes to indicate an alarm.
The neat thing is that on a four hour flight you have that information as soon as you settle into cruise, so you have plenty of time to plan your fuel stops if the winds are not as forecast. But if the winds change, so does the forecast reserve.
Besides that you get fuel used for each side, miles per gallon, etc. Once you are at cruise this predicted reserve is quite conservative since you will burn less fuel during your descent.
The unit is extremely accurate, easily to within less than a liter when filling the tanks, so if you start with full aux tanks (and tip tanks if you have them) you can burn each tank down to just almost empty without risk of having that sudden silence that can sometimes upset the passengers.
I know that JPI has several new units available, you probably want to talk to one of their people about that on the phone, as I think their website is lagging behind a bit.
Jay