20-30%. I like the extra battery life. If it's getting really low and I still need to do something before I get back to a charger, I'll cut the backlight completely.
I'd say somewhere between 30-35 percent. I can't imagine using the iphone at 100 percent though - perhaps in direct sunlight? But 100 percent is DAMN BRIGHT!
Mine was at 100%; I guess I'd turned it up that high for some reason and then forgot about it. My eyes are used to it now. I just turned it down to something like 70% and it looks freaking DARK.
How bright is the sun? I try to keep mine on that level all day every day. That way when I throw my suit jacket back and a slowly slip it out of the side holster in a crowded office elevator full of Crackberry junkies at 7:30 AM everyone knows I have an iphone
100% - I'm not stupid, I have a Mirror Screen Cover
I'd say I get about 2.5 hours of constant usage, (ie. playing music, surfing web over WiFi/3G, using really power intensive apps, push on, fetch mail on.)
70% most of the time though I crank it to 100% in direct sunlight. If I'm in a dark area like a bar or club I'll take it down as low as 20%. I think its kind of awkward having a bright glowing iphone in the middle of dark bar announcing your presence. It ruins the mood.
i think it depends on where you live. I live on the beach in alabama but spent the summer in maine, and i noticed that anything lower than 100% in alabama was way to dark but in maine i only had it around 30%.