I just recently discovered Redux. It all looks good. Is there any downsides, gotcha or compromises from using Redux instead of Flux? Thanks
Redux author here!
I'd say you're going to make the following compromises using it:
You'll need to learn to avoid mutations. Flux is unopinionated about mutating data, but Redux doesn't like mutations, and many packages complementary to Redux assume you never mutate the state. You can enforce this with dev-only packages like redux-immutable-state-invariant, use Immutable.js, or trust yourself and your team to write non-mutative code, but it's something you need to be aware of, and this needs to be a conscious decision accepted by your team.
You're going to have to carefully pick your packages. While Flux explicitly doesn't try to solve “nearby” problems such as undo/redo, persistence, or forms, Redux has extension points such as middleware and store enhancers, and it has spawned a young but rich ecosystem. This means most packages are new ideas and haven't received the critical mass of usage yet. You might depend on something that will be clearly a bad idea a few months later on, but it's hard to tell just yet.
You won't have a nice Flow integration yet. Flux currently lets you do very impressive static type checks which Redux doesn't support yet. We'll get there, but it will take some time.
I think the first is the biggest hurdle for the beginners, the second can be a problem for over-enthusiastic early adopters, and the third is my personal pet peeve. Other than that, I don't think using Redux brings any particular downsides that Flux avoids, and some people say it even has some upsides compared to Flux.
See also my answer on upsides of using Redux.