I have both the lighted and non-lighted cover arriving very shortly (on the truck for delivery), so I'll update here what I feel about both. So far I use a clip-on light.
For those that say the battery drain should be minimal, that really depends. My Mighty Bright uses 3 AAA batteries, and only lasts around 20 hours before they die. And those are LED bulbs. So I actually would expect the Kindle 3 to lose at least half of its battery life with the light on.