I've ordered one for the GS (not GSA) - for road riding, the location/size of the rear brake is useless and I have to admit to becoming lazy with the cross-linked brakes (i.e. I mostly only use the front).
This is a bad habit to get into but, without the extender, I don't find the rear brake at all easy to use.
I'm sure that any of the brake extenders available will make a hell of a difference - after all, manufacturers/suppliers would not sell them if we were not all buying them - something else that BMW should look at in my opinion.