I have one. Was one of the first to actually.
Last I heard, RKSport discontinued producing them, due entirely to lack of sales.
They were originally sold exclusively for the MKS. However the rear windshields between both platforms are identical, so I installed it on my SHO back in like 2011.
I get comments on it periodically, as some notice it as being a mod. Other, less knowledgeable spectators simply think it came with the car as an OEM item.
No real benefit as far as I've been able to tell, other than just aesthetic appeal.
Rear window doesn't stay any cleaner. Plenty of pics of my car floating around, so umm yea.