I have been using bootstrap for years and generally prefer it as the starting point on all of my front end designs. But I'm working on a project right now with another developer who doesn't want to use it because he says it's heavy and doesn't follow best design practices.
I have never had a problem before. What do you think? Why not use bootstrap?
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