I'm migrating from RethinkDB to FoundationDB right now. One thing which is easy to miss when you look at FoundationDB is that it is more of a toolkit for building databases than a database in itself. So, if you're looking for a faster horse, say a "better PostgreSQL", or a MongoDB that doesn't lose your data, it will seem disappointing ("where are all the features I expected?").
Where I think it really shines is when you couple it tightly with your application. Suddenly you get a fully transactional distributed database with clearly understandable semantics, and if you can show some flexibility in how you map your data onto the underlying primitives, you can get fantastic performance, as well as correctness.
Where I think it really shines is when you couple it tightly with your application. Suddenly you get a fully transactional distributed database with clearly understandable semantics, and if you can show some flexibility in how you map your data onto the underlying primitives, you can get fantastic performance, as well as correctness.