I just received an entirely welcome email from Amazon, which boiled down to "we wish to give you three free things a month". OK, not quite free. But close.
I sometimes review stuff on Amazon, and people, in the main, seem to find what I say in my reviews to be fairly useful. People can vote on a review to say that they find it helpful or unhelpful, and I've got something like a 97% "helpful" rating. I'm moderately confident that the couple of people who have said "unhelpful" simply disagreed with my critical reviews of their favourite bands' latest releases.
Anyway, it turns out that once you reach a certain threshold Amazon checks your postings and might invite you into their Amazon Vine program. In Vine, you get sent a catalogue or products - books, DVDs, games, electrical goods etc - every month. If you see anything you fancy, you can order up to three of those items and they are sent to you, completely free. This happens every month. Amazon, in return, asks that you review the items, and promises that your review - good or bad - will be spotlighted, without editing or alteration.
Given that I like free stuff, that I enjoy writing reviews, and that I can knock out an edited five-hundred word review inside 40 minutes in my lunch hour, I fail to see the immediate downside.