I've been using bootstrap for a while now too and lately I've been trying out new libraries just to see if anything is better yet. On feel alone I didn't love foundation.

For a project that required pretty heavy customization I ended up using skeleton/react (http://getskeleton.com/) and enjoyed it.

I saw Semantic UI on here the other day and I may end up trying that out on my next project: http://semantic-ui.com