Flux is very boilerplate-y, even with some of the lighter frameworks. But boilerplate isn't particularly complicated, so you get over it.
If I were going to a hackathon I would not use flux+react, I'd probably go with angular. If it's something I plan on maintaining over a long period of time, react is currently my big choice.
Testing react is very nice, especially coming from angular where testing directives was very painful. (And testing everything else was needlessly verbose.)
I made a VERY simple project starter  that I'm going to take the chance to plug now. It's built for frontend web applications in mind, and it focuses on giving you the starting environment to build an app. It provides development, testing, and production builds. In development you get live-reload and hot-loading of assets. In production you get minification, concatenation, and cache busting of assets, as well as getting all assets copied to your output folder. There's a live testing environment (i.e. watch files and run tests) as well as a single-run environment. Tests also generate code coverage reports. The best part is that it doesn't use gulp or grunt, it's just a webpack config with a few flags per environment type, and to run every task you just use npm run-scripts. Out of the box it handles ES6/ES7/JSX, CSS, images and fonts, but extending it to support other assets is trivial. I took the time to heavily comment the webpack config in order to help people out.
EDIT: My coworker pestered me to edit this and mention that we're hiring . If you're interested in using React and Flux, reach out!