If, like me, you're thinking of opting out of the care.data project, Ben Goldacre's reflections make for recommended reading. His overall position is that the concept is sound (and indeed vital to making progress in medical research) but that the government have made a colossal cock-up in the way (or various different ways) they've tried to sell it to the public and in the lack of robust safeguards. However, opting out for whatever reason would be damaging to the value of the project, which is dependent upon the massive scale of the available dataset, and "an opt-in system would be worse".
By delaying the project by six months, the government have bought themselves time to get it right. Goldacre's advice is to hold tight and see if they do before making a decision one way or the other. One senses that he's not particularly optimistic.
(Thanks to Simon for the link.)