Not great in direct sunlight. Unfortunately, the screen requires a backlight, which means it's not reflective, which means ambient light washes it out.
That being said, it won't be any worse than most colour LCDs out there. I have one of these and they'd make a great carputer. Rip out the WiFi board and install a crystal HD decoder and you can even play 1080p HD vids. Power consumption is very low as well. And you can find special made car power adapters for them at a very low cost. Main best thing is that they use a lower voltage than the car voltage, making power supply design simpler (and therefore cheap power supplies less likely to blow up).