You can read the developer documentation here: https://docs.mendix.com/releasenotes/studio-pro/9.0/
The blog with more marketing content is here: https://www.mendix.com/blog/mendix-9-0-where-it-all-comes-together/
From my experience, the upgrade is def. worth it.
So much great improvements to the developer experience are added in the way styling is handled, page parameters, etc.