I've just picked up a wind stop on ebay. I wanted to try it to see if it made much difference to the rear passengers as I have the kids in the back with the roof down quite often. Given the comments on here and other places I didn't have high hopes but I'm really surprised at what a difference it does make.
I've only tried it on my own so can't comment on the effect it has in the rear but I imagine it will be quite a drastic difference.
I first tried the car without it. I don't think the 500c is bad at all for buffeting compared to other cars I've had but there is some especially over 50. It was only 4 degrees so I felt everywhere it was blowing!. The wind blows back around your head and neck. It also blows around the back of the seat, under your arms, across your lap and down your legs into the foot well. At 60-70 it's quite a gust coming back between the 2 front seats.
With it fitted you still get wind around your head and neck but about 50% less than before. The surprise was that lower down any buffeting has virtually disappeared. I would say at motorway speeds it's similar to driving without it at 40.
I was also worried how intrusive it would be (or rather how silly it would look) but you hardly notice it. In fact from inside it hides the folded roof so improves the view. It doesn't extend above the folded roof so can't be seen from outside and it doesn't restrict the rear view anymore than the roof does. When the roof is up it does cover some of the rear window but as you can see in the photo it's quite transparent.
It's also really easy to fit. It took longer for me to squeeze into the back and lower the rear seat than clipping it into place. It spring clips behind the bracket and hooks under rear shelf, the rear seats then secure it in place. Removing it seemed to be a bit fiddly but I'm sure given a few goes it will be easy to find the knack to it.
The other bonus is it's small enough to store easily, I keep it in a cupboard indoors, it also fits into the boot if you did want to remove it while you are out and about. I can see myself fitting it again in the spring and leaving it there all summer.
The £100 new price is steep for what you actually get but I think it will make a big difference if you have the roof down often.