I use my own library extensively (which has been on HN few times), mainly because since I made it I know every bit of it. But I use it as a base to grow the style and have decent ground such as buttons/inputs/etc.

So I only need to take care of the bigger things like colors and shapes, while the library makes sure the small bits are consistent (widths and heights, box-sizing, :hover and :active, etc).

I've tried not using it but the amount of work required to make CSS work and look nicely with all those quirks just doesn't make sense. It's much easier to overwrite a background and border-radius than to write a proper button from scratch.

Next step, I'm thinking about making it all abstract with scss then I can just @extend those %placeholders.