Glare will always be a problem with the top down; with the sun behind you at certain angles it's worse than other places but if the unit is in shade it's usually pretty good. The same problem exists with LCD gauges.
Most Android head units are made in China so use cheaper screens which doesn't help.
Adding anti-glare film to these units versus the brand-name units is beneficial (brand-name units use better anti-glare glass).
I tried adding shrouds but they needed to protrude too far to make it worthwhile (and looked shite).
Having said all that, I'm a huge fan of Android head units in an MX5, the extra functionality they can provide is fantastic, Chinese units are cheap, and there's no point in putting a high-end head unit in a convertible anyway.
If you do purchase one, here's what to look for - the 2nd point is significant:
1. Get one with the largest resolution possible otherwise the App's buttons/labels/decorations/menus will consume too much real-estate (all navigation Apps are bad for this, they obscure too much of the map if your resolution is 800x480(WVGA)). When I bought mine, 1024x600 was the largest resolution available for Chinese units and it's fine for navigation apps (but larger is better!).
2. Be wary of Chinese Double-Din units whose front bezel fits over the housing and is secured by screwing into the sides of the unit since the unit will slide into the Double-Din slot and stops at the bezel. If you can somehow figure out how to get the unit to lock into place when you slide it in, there's no way to be able to get it out again!
See
http://mx5cartalk.com/forum/viewtopic.php?f=57&t=41091&start=143 for my 2 installations and comments.