Even without the flow analysis and better typing, incremental compilation is a huge improvement over TypeScript, which re-parses type declarations on every compilation. That quickly leads to large compile times when you have type definitions for third-party components (even though you may only be using one definition in the file, the entire definition is parsed).

The existing available definitions from the DefinitelyTyped project is a huge productivity booster. Apparently Flow also has similar .d.flow files, but it will probably be a while until they exist for common projects.