I have installed them on main wing of my Integral and the overall performance has been improved. My Integral is quite porky (at 5kg) and before VG installation there was a little wing rocking on horizontal lines.
After VG installation I have noticed that the wing rocking has gone and it feels more stable in the air; its penetration has been improved, too. Also, wing tip stall has almost been eliminated.
I cannot see any significant difference in the drag. However, spin entries are quite a challenge now and they require a lot more elevator

To be honest I wonder why not more F3A fliers do not use them.