Good. No complaints, no issues. Don't accidentally hit the Select button until you understand what that means... They've placed it very prominently, so it's easy for new users to hit by accident. Basically Select means you get some benefits back from Amazon, like being able to make your book free for five days out of every 90, in exchange for exclusivity on Amazon for 90 days (and it auto-renews if you don't turn it off). Mixed feelings about that one: can be nice for promo, but means no sales on all the other retailers, so it can be good, can be bad. In general, I want to have most of my work available on ALL ebook retailers.
I've made over $1100 through KDP in the last two months so I kinda like them. The free promos are helpful. And despite what they say, they DO pay you a small royalty for promo copies, though how they calculate that (or anything on the royalty statements) is pretty incomprehensible to me. Amazon math is an arcane science and I have no mastery of it
But yeah, you have to be exclusive ... no Smashwords. From what I've heard from other authors, most usually get the majority of sales from Amazon anyway.
Stressing again - know the difference between KDP and KDP select.
All my books are on Kindle Direct Publishing, and I am happy with how they operate. Just make sure you read all the agreements and such - remember, putting stuff up there for sale means you are going into business! It's important to understand the details of the arrangement.