My experience with a JPI-700 engine analyzer has been most rewarding.
I have all CHTs, EGTs plus fuel flow and interfaced with the GPS.
All kind of alarms for temperatures, shock cooling early advise, battery voltage, fuel remaining. There are some options like oil temperature and external temperature. The EDM 800 model has also HP, manifold pressure.
There is no connection to the fuel tank gauges so the instrument cannot read the fuel level directly. However, this has proven not to be a problem. If I fill up, the total capacity is already stored in the instrument setup so it is just a matter of keying "full tank amount".
Then the fuel totalizer always takes care of the fuel used and remaining. If I add only a few gallons, I just manually add the amount. I can also deduct any amount if I take out fuel from the tanks. Easy and almost second nature.
The GPS link allows to have a real time display of: fuel needed to the next check point, the remaining amount and the reserve at the destination, plus actual fuel flow and miles per gallon, which is the key value.
I have a Raven I and I can fine tune the EGTs with carb heat and manifold pressure to minimize the spread down to 40?F, to improve the fuel distribution to even better than an injected engine. I can usually get better than 7.5 NM/Gal.
JPI has a good deal of the EDMs with the fuel flow option. IMHO, the best bang for the buck, never a problem in now 3 years, accuracy is outstanding, better than 1/4 of a gallon in a 3 hours trip.
I do believe it should be a basic instrument in the helicopter. By the way, never a problem with stuck exhaust valves as I can cool down till having 0?F/min plus knowing the actual CHTs. All exhaust valves tolerances are in the middle of the engine specifications.
My 2 cents.