I have VMG and it is amusing when on a long tack, like when sailing 20+ miles from the coast to an island.

One thing that you might not think about until you actually use one:
Let's say you are heading toward a target that is 1 mile directly upwind of where you start from. Your VMG will change, each time there is a wind shift. Also, as you get closer to the target, you will only get good VMG readings if you are nearly on the lay line for the target. The closer you draw near your target, the worse this becomes. The target is rarely near your 12 o'clock position and becomes farther and farther away from the 12 o'clock position as you draw closer, except when you are on a direct (final) approach (sailing right on the lay line).

When starting a mile away, the target is not more than 50º off your bows but, when you get closer, the target may be as much as 100º off of your bows and this turns VMG into a negative number.

You can try to compensate for this by extending a line beyond the target, and projecting a waypoint well beyond the target. But each time you begin a new tack, you would need a new projected waypoint. And each time the wind shifts, so does most everything else.

For me to have an accurate gauge of my upwind performance from an electronic instrument, I would need to know my VMG to a projected waypoint that is projected a constant number of miles beyond the target, in the exact current bearing of the target. A computer can provide this, but the computer will need to know the apparant wind speed and apparant wind angle, combined with boat speed over the ground, location of the boat, and location of the target in order to extrapolate
such a number.

Personally, I think that the most usefull reading, upwind on a catmaran would be apparant wind speed; keep it as high as possible! I want a very faint audio pitch (in my ear from my sunglasses frame or helmet) in my ear that varies in pitch as the wind varies in speed. ("Electronic Shroud Whistle")

GARY