I have seen Material UI. But still looking for an easy to material design framework. Something that would keep the idea of Twitter Bootstrap will be more than welcome.
I used the Material UI in a small project I did with some friends. It worked really well, and we were able to build it quickly for the mobile web as well, which is a big selling point for the Material Design UX. My favourite part about it though, is the really cool loading icons. I've always loved Google's loading icons, now I get to use it lol.