I have used SemanticUI extensively, it has got a lot of great components and you have got lots of customization options. Apart from that you can checkout Ant Design for react as well.

Justin Friebel : I would like to draw you attention towards Grommet as well. have a look : grommet.io/docs/icon

My primary UI Kits I use is Bootstrap 4 for medium or complex projects and spectre.css for lighter ones. Really depends on the project. I used semantic Ui with vue.js in a Medium Project and was pretty disappointed. Horrible render performance on slightly larger data sets and the grid is honestly crap.

But always looking out for Ui kits with good customization capabilities

For those following this, I've started to implement Rebass into my project. It was very easy to start using with create-react-app, no ejecting or custom webpack plugins etc.

It gives you a good minimal base and uses styled-components for easy customization. So far I am really enjoying it! If I hit any snags I'll let y'all know.

jxnblk.com/rebass

I have been looking at the Ant Design docs, it seems to have some promise. Anyone have experiences to share about it?

I've been really digging Semantic UI. They have a React version you can use.

I haven't personally used it for React yet, so I can't speak on how refined it is, but the jQuery version works really well.