I tested Premium on Leo on my small account - the one where I write poetry. It is giving you a slightly more exposure, and Leo curration account usually is geting you at least a couple of votes that months to be sure that you get some value back. I would say that it is quite useful for small accounts. If you can afford it.
I have no idea about the curation that goes on behind it, but I assume that they are supporting some of the content at least. I hope that they support those who drive value, or at least attempt to, rather than those continually extracting and putting little back.
Yes, they are quite good. I think they got 4-5 curators, for different specific content.