I've used my iLiad for more than 5 hours straight fairly often. Since classes in my doctoral program only meet once a week, they last 2.5 hours each, and some nights I have two classes back-to-back. I use my iLiad to take notes. If the battery only lasted 5 or 5.5. hours, I'd have trouble.

I also use it at all-day conferences, though less often. I can take notes on it for 8-9 hours before it's out of juice. (This is the original model, before the battery expansion.)