I've been very impressed by Jazz -- it enables great DX (you're mostly writing sync, imperative code) and great UX (everything feels instant, you can work offline, etc).

Main problems I have are related to distribution and longevity -- as the article mentions, it only grows in data (which is not a big deal if most clients don't have to see that), and another thing I think is more important is that it's lacking good solutions for public indexes that change very often (you can in theory have a public readable list of ids). However, I recently spoke with Anselm, who said these things have solutions in the works.

All in all local-first benefits often come with a lot of costs that are not critical to most use cases (such as the need for much more state). But if Jazz figures out the main weaknesses it has compared to traditional central server solutions, it's basically a very good replacement for something like Firebase's Firestore in just about every regard.