I keep it on lowest almost all the time. The battery lasts longer but, more importantly, most of the time my DS gets used the most as I'm falling asleep. If the light is too bright it hurts my eyes and wakes me up too much. Honestly, I wish there were a dimmer setting. Forget brighter screens, I want dimmer ones in new consoles. XD;
I have had to set it up to high the last while, though. :[ If I play my DS on the way home from work, the sun is too bright and I can't see a thing baaaw.