Susy is a lightweight grid-layout engine for Sass, designed to simplify and clarify responsive grid layouts without ever getting in your way. You can use Susy with floats, flexbox, tables, or any other CSS technique. You’re the expert, we’re just here to make your job easier.
I like the idea of grids-on-demand, rather than a strict framework.
My experiments have left me impressed. The current state of CSS layout means that unless you like to spend a lot of time doing calculations something like Susy is really useful. The output CSS is pretty much what I’d come up with myself, which to me is the acid test for tool use.