I have a Garmin Edge 800, or rather Garmin has it. Second unit in 18 months and it has gone back to them. Anyway during the day commuting I tend to ride with the backlight set to about three bars but on tour or on Audax rides where I need to last longer I turn the backlight off. It is okay with the backlight off but better with a couple bars of backlight. If you plan to do long rides (say over 12 to 15 hours between charges) I guess it would be advantageous to go with the backlight off.
Oh BTW I understand that the 2012 Christmas in Garmin land will be a special one. You might want to hold on the buy decision
Andrew