Well here's what I have found so far:
The Wiz's screen is great in daylight, but not so brilliant with direct sunlight shinning on it, all you need is a little shadow and then you can see the screen again. The one thing I used to find with my GP2X was that if I got reflections on the screen I had to tilt the 2X slightly but then because of the poor viewing angle I then couldn't see the screen, but obviously the Wiz doesn't suffer from this as the viewing angle is amazing, in fact I haven't seen any LCD screen to beat this OLED one yet.
I have used my Wiz in a multitude of different places including while I was waiting for a taxi outside a shop on a sunny day, and it was perfectly visible.