I've had the GRT AoA running on my EFIS for as long as the software has been available, and have posted quite a few times on it. I have since also installed an AFS AoA Pro, wich is sensed and sits on the glareshield.
After testing both systems against each other, I can say that if they are calibrated the same, they will both give good indications in the pattern under what I would call moderately steady-state flight. The GRT really doesn't keep up during aerobatics, however, while the sensed unit does quite well regardless of attitude. I also find the glareshield mount to be superior to the indications on the display due to field of view on approach. An audio component to the GRT would make it a great deal more useful, and that shouldn't be hard to add.