I have played with Susy, Bootstrap and bootstrap-sass, foundation, skeleton, UIkit. Every time I revert back to bootstrap, only because I have the grid memorized. That and I am just now getting into SASS and bootstrap SASS is becoming something I am comfortable with. Which is a strong statement because I am a slow learner!
My experience is very good. You can build fast new ui´s with a very nice css syntax. When I started I sometimes missed to have no bootstrap because many angular plugins are based on bootstrap.
Kleo Petrov
Professional human being for 29 years
Semantic UI, as Lars suggested. I've used it in a couple of projects and was really impressed by it.
Another framework worth mentioning is UIkit. Although not that popular, UIkit is still very powerful and fast.
You can also check some of the minimalistic frameworks like PureCSS and Skeleton.