The subscription costs a dollar a month (a cost that Amazon apparently sets, because they never asked for my thoughts on the matter) and if I'm reading the fine print correctly I get ~33 cents on the dollar. So that's $4 per person per year if I'm adding that up correctly. Note that all you're getting is blog content -- updated on the Amazon servers every 15 minutes, I believe -- and not the thread comments.
Bloggers in the audience who would like to sign up their own blogs for this service can sign up here. (All you need is an RSS feed address.) The only reason I found out the service existed was because I was looking for "Comics Curmudgeon" on Amazon.
I've tested the subscription on a Kindle Paperwhite -- a recent gift from a relative on the grounds that the training I have to take for work comes only in Kindle e-book form and I couldn't heft the heavy paperback versions -- and it looks pretty snazzy. So if you like this sort of thing, check it out. Note that the "supported devices" seem to be only the e-Ink readers; it doesn't look like the Kindle Fires are on the list. (Which is kind of weird, if true.)