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Mine was probably split pretty equally between both. I usually had some work to get done, and then would consider it my time. Even if there was not anything pressing, it was a great time to do the mundane stuff you hated doing in the office like reorganizing your e-mail or updating stuff, finishing presentations, etc you didn't need to be online to do at the time.

I've never worked for a place that in any way expected you to get work done on the airplane, so it was always my choice. I've always felt it was way more productive at times then at my desk.