I don't like losing access to things. If it was "you can download one per month and keep access as long as you're a subscriber" sort of the way Playstation Plus works that would be OK with me.
But the rental program feels more like a cruddy crippled library than a real service.
I'd rather pay the $5-$10 for the eBook and not have to deal with it.
This new service is interesting. I don't read enough for it to be useful, but I could see it saving my dad quite a bit of money (or trips to the library) if they had enough books.