Slightly unrelated but still, I recently had the opportunity to drive an Innocenti 1300 Export for a week while on holiday in Italy (pretty cool!) and of all the things about these cars, I was most excited to use their quarter windows with the think that I would be able to fine tune the amount of air or, better direct the air. Nope! Like mentioned above, there is a low pressure area, I assume, caused by the shape of the front and not helped by the body seams. There is literally NO air flow there...rending these the most Italian additions ever...purely there so you can smoke:-) Ha. 

I'm going to give a review of my plexiglass install. Of course this is an unbiased review.

First, the 2 additional plexiglass panels work. I don't get the constant wind buffering on the head. Sometimes, I'll get a gust of wind across my face which is refreshing in this hot summer weather.

Second, The sizing of the plexiglass is perfect. When the window is down, there is still enough room to rest my elbow on the door. Any larger, I would not be able to. I left about a 1/2 inch gap between the door frame, so there is a tiny amount of air coming in to cool the back of my head and neck.

I'm giving a thumbs up for this inexpensive improvement to the ride quality.

Sounds like you got it figured out. good job. Pics?

So far I'm very happy with my findings and results. For $10, I bought two pieces of plexiglas 15" X 7" . Curved the upper corner to follow door curve. These two pieces are going to act as the rear glass of the sliding window. I used foam double stick tape for attachment to the inside of the door frame. Like the adjustability of the sliding windows, I left a 1/2 inch gap from the rear from to get a little airflow. 

Gone is the constant buffing to the head and the hair going forward. Occasionally there is a breeze to the face, if you want more 6 inch or shorter will get you there. Now I can ride around with the windows completely down.

Thank you guys for the response about the wind deflectors. 

Well, I'm game to subscribing to your efforts. Please post your results of your experiment and also if you happen to purchase the deflectors and what the final word is on those. 
Being that the Mini is smaller in every way in comparison to the rest of our vehicles that have the deflectors, I wonder how well they would work. My head is closer to the headliner than the other vehicles. So I was thinking of opting out on them. I'm 6'2". How tall are you, so we know how that factors with the results. 

I've been experimenting with some cardboard. The "B" pillar and door frame forces the air to baffle the head. The rear sliding window blocks that buffing when it is positioned properly. I'm going to make a plexiglas rear window and attach it on inside of the door frame. We'll see if that works.

I know that you're specifically looking for replies from people with them on Minis. But I'd thought I'd share that I have them on two other non-Mini vehicles that we have right now. 
In answer to your question regarding stopping the wind from hitting your head, it would depend on how tall you are. The taller you are, the less it will affect you or your passengers. But for the most part, it won't stop it. The air/wind will flow around the defector and into the cabin and circulate it around inside, the further the window is down. So whether it's direct or indirect air flow, you'll still get it. Unless you're only cracking the window down a few inches within the coverage of the deflector. Then you'll be safe from the wind-swept look. 
They deflect wind, rain and bugs to a certain point. Once the window is rolled down passed the deflector, it's marginally different than if you didn't have them at all. It all depends on how far the window goes down. But there is some difference. You're not getting nailed as hard from the air coming off of the A-pillar. 
They assist in the way of wet weather also. Being able to have the window down a few inches without getting rain in. 
We mainly got them for our dog. So that we can have the windows down a few inches without having a major blast of wind and/or rain come in, and still have the air circulate naturally. There still will be wind noise regardless of how much you have it down though. 
So in all, there is some notable difference than not having them. They provide some shelter from the elements. 
Having the pop-out rear-side windows may either help or hinder the desired effect with cross flows as well. 

Those wind deflectors are good for about 2-3 inches of window down... or leaving them cracked in hot / rain time.
  They wont cure any buffeting when the glass is more than halfway down.

Agreed, the sliding windows managed air flow much better than the roll-ups. My first Mini was a Countryman - it had sliding windows in the back too.

I happen to like the sliding window system. You can get a nice cross flow by setting up the window openings.

If no one answers the question, I am going to make  a cardboard template to see the effectiveness of blocking the wind. 

I have not tried them, but here's a couple of my thoughts re Minis with roll-up windows.

At any appreciable speed, there is a low air pressure area on each side of the car just behind the A-pillars. If you have your roll-up windows open a crack, air gets sucked out of the cabin. This shows up if you have exhaust system issues - the reduction in cabin air pressure sucks exhaust fumes into the cabin. (You can also get the same effect if your forward sliding windows are open an inch or two.) If you drive a Mini in the rain, you will probably see drops of water on the window glass where the low pressure eddy keeps them from streaming away.

So, I think rain is not likely to come in along the forward edge of the roll-up window. I suspect the deflectors might exaggerate the low pressure effect and would only keep rain out when the car is traveling slowly or stopped.
They would probably afford some protection along the top of the roll-up window.

Personally, I don't think they would be worth it.

Hello all. I have a question for those who have purchased and are using the Wind Deflectors MSA1102 for the roll up window minis from our host. Do they work? Do they stop the wind from hitting your head and hair while you drive?
Kinda pricey, so I  thought I'd ask. Thanks Jim