Currently, Scribd pay full purchase price for a read. Amazon don't pay full purchase price for a borrow. I'm surprised that they have auto-enrolled people without telling them the terms for this, unless they're planning to pay the same as they do for a borrow. Though, how they figure that out if the way of calculating the pot split has changed, I don't know.
I'm up for multiple outlets, so if it were open to non-select authors then I'd sign up. But since, to be honest, I can't think of subscription services without thinking of musicians being paid cents (or fractions of cents) per play, subscription makes me a little bit twitchy. So, I'm definitely not going to go exclusive for it!*
I'll be watching this with interest though to see what it does to the market.
*To be honest, I would avoid exclusive like the plague anyway. To give one company 100% of your business is to give them 100% control over what they decide to pay you!