There is so much to learn out there when it comes to using HTML5 and CSS, so it's always a bonus to have a nicely compiled framework to get you started.
So far, I've tried Twitter Bootstrap, PureCSS, and Skeleton.
Bootstrap has some amazing features and functionality built in. When you get the terminology right your work can really flow. One downside to Bootstrap is the size, but if it's what you need, it's totally worth it.
As for Pure, it's slightly less complicated (but doesn't offer as much functionality as it is pure CSS). Pure supplies nice clean coloured interface selections broken up into modules so you can select and use certain parts (buttons, grid, forms, base) of this framework without worrying about the bulk weight.
Skeleton has been my favourite thus far. What makes Skeleton so nice to work with is the absolute simplicity. It's meant to be styled over, and the Skeleton.css is so beautifully written, it provides you with a list of specs, which have proven difficult to find for Pure and Bootstrap alike. Although Skeleton is the most basic of these, it is built upon normalize.css, and offers forms, standard styles, 960 grid system, buttons, and everything is black and white, so you can really go hard and customize.
Breakdown:
Bootstrap: Functionality, very capable, many options and plugins.
Pure: Smooth and clean, easy to make a mock up in half hour, modules.
Skeleton: Built to be modified, simple, small, well documented.
So take your pick. Each of these have been a pleasure to work with and you can get some really amazing results while minimizing your workflow.
