I actually like reading on it better than reading on the Fire HD 6, even though the resolution is not as good. I don't know why I like it better. Maybe the size, maybe just because it's new.
However, I have experienced an issue with Kindle Unlimited books and the Fire HD 8. Multiple times! Attempting to open a book (which has been opened previously, and is still an active book), I get a message that says "this book has DRM and can't be opened."

The only option is to delete and download again. Quite annoying!