Only time I had an issue with it was when we were driving into the sun coming back from Balloon Fiesta this past weekend from NM to AZ.
My polarized sunglasses make the screen disappear. I also hate looking at the few specks of dust inside my iPad that glow like beacons when the sun shines on them.
Normally for limited use in the sun, I have no problems with the iPad, even with the brightness at half. For playing a game, studying, or reading for any length of time, it's not as much fun. Even my Nintendo DS was terrible in the sunlight too -- it's just the nature of the screens in general.
So instead I had to go "old school" and read a couple of books I picked up on the Painted Desert and Petrified Forest. Learning is fun! 😛 I was going to sell my old Sony Reader, but I might just keep it around for road trips when the sun is in my face.