Hey Rod, how ya doing. Only negative thing I have heard about them is the flex in the arm which allows the mirror to move/shake a fair bit more than a windshield mount. But it does have that certain bling...
I just purchased the tower mirror that is made by Boss Accessories. They can make any diameter you want to fit your tower. I think it looks better than the Mastercraft model. It doesn't flex but if will obviously shake when the tower moves.
Rod I have the Mastercraft tower mount mirror and it doesn't shake at all, there is absolutely no flex in it because it doesn't have a joint as the boss one does which makes the arm weaker. My dealer said a lot of Malibu owners were making some kind of shim that would fit around the tower to make the mirror mount fit. The arm puts the mirror in the perfect position while your driving-above the windshield enough that you can still look over the windshield while driving. I love it.
I also have the MC tower mount mirror. No complaints at all. It gives the driver a perfect view of the rider and doesn't obstruct your view for driving at all. It is rock solid and as an added bonus looks pimp.