I've got it. Like the Kindle itself was back when it first came out there is a hit or miss quality to what is available but a decent selection is there.
A bunch of older titles, like pre-kindle existance older titles, that have only been recently appearing on the kindle are in the Unlimited. This is fine for me as I have read some of them when they were 99 cents but don't think they are worth $5.99 each and I don't have the patience to wait a whole month before I can borrow another book. I read 10-15 books a month and easily spend more than $10 doing it even though I stick to the library or spending under $3 for 95% of them.
But new titles or popular authors that I am willing to spend $10 a book for are not there. Yet anyway.
It meets my pages to cost ratio also and solves a problem for me there. I won't pay $5.99 for 130 pages of anything. Or even $2.99. I'm barely willing to pay 99 cents for that few pages. A bunch of books I'd like to read want $2.99 to $6.99 for 130-18 pages and I'm much too cheap to pay it, even though I want to read the books. Most of them are available on Unlimited.
I like that I can 'borrow' more than one book at a time. I can get all 5 books in a series downloaded and blow through them without pause.
It is tricky online to just search for Unlimited books but from the kindle itself I find it easy. And if you are just searching for a book and that book happens to be an Unlimited one, then the "Read for Free" button just appears anyway.
And there are free Audible books! If you are a member already you get 3 free credits added to your account, if you aren't already an Audible member you get 3 free months.
I don't know if I will keep it long term, that depends on title availability, but I can see getting a couple months out of it right now