I really like the composability of Reakit as that solves the issue a lot of UI component frameworks tend to have in not being flexible enough. Although personally the only React one I used so far was Material-UI v1.
However, I think it's better in the long term to create your own components and use primitive helpers like Downshift to reuse complex behavior with your own styling applied.
I used Bootstrap because it felt really flexible and then, when I started using React, I switched to React-Bootstrap, but it didn't feel as powerfull as plain CSS-Bootstrap, so dropped it again and did my own components with CSS-Bootstrap.
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