My new kindle and I were on two BA business class flights in the last couple of weeks. On the first, the take off rule was ignored by one pax (not me !) and equally ignored by the cabin crew. On the second it was politely enforced by the purser. Once the cabin crew left their seats to commence service about ten devices appeared without objection.
It is important though to ensure that the wifi and 3G are off. Easily remembered for one's phone but equally easy to forget on the book.