Here's my incidence meter opinions:
I purchased both the Great Planes and Robart Incidence Meters.
I like the Robart for the bubble level but it doesn't seem centered with the needle which is hard to see unless your work area is really bright. It is excellent for determining level surfaces though and it is better made. If you use it without the "wing clamps" and level the plane and then put the clamps on and put those on the plane it's balanced both times
The GP is way more accurate but it mechanics have a lot to be desired. I have to tap on the table to get it to center because the pivots are too tight. There seemed to be some variances if you put just the meter on the level surface and then put the clamps on the surface but as accurate as it is it may be a lot easier to see just a little deflection. I might try adjusting it to be equal.

That's it. This is the first time to use them but they sure take the guess work out