I've been in what could be a "worst case scenario" for screen visibility, and it was definitely hard for me to see but still visible enough:
* I get into the car in my garage, and the ambient light inside is dark enough that the screen comes on in "night" mode.
* The sun is out but relatively low, near the beginning or end of the day, so after I open my garage I put on my polarized sunglasses.
* As I back out of the garage, the sun is at an angle such that it glares directly onto the screen, not sure if it's through a side/back window or the pano roof.
* The screen doesn't immediately switch to "day" mode, so it remains dim. Once it does switch to "day" mode (usually within a minute or so) I'm fine.
Any other time, I've had no problem with the screen being too dim. I sometimes find it too *bright* at night, but that's a simple adjustment with the right-hand thumb wheel.