I have had two sidestrings on my Pilatus B4.
I works fairly okay, but the location was a bit tricky in my case. In some cases it would not move at all (except during tail-slides :-)).

Here is a video of a spin with my setup:
https://www.youtube.com/watch?v=RWPya_vchDQ

You can see the vortex on the right-hand side which is generated by the high yaw rate of my glider.