The ability to use the responsive flexbox classes has been game changing for me. I can do large portions of a fully responsive flexbox layout with css classes like d-md-flex and flex-md-row, etc. This drastically cuts down on the amount of stuff that needs to be in my styles.less or whatever. And it makes maintenance easier for that reason as well.
To finish the layout I generally am making a few utility classes to control things like color and spacing (not a fan of the default bootstrap spacing classes). And then of course custom CSS for navigation, typography and other misc widgets.
But the bulk of the main layout is handled entirely with bootstrap classes. And because nearly everything can be specified with the responsive breakpoint prefixes, I don't have to write repetitive media queries.
To finish the layout I generally am making a few utility classes to control things like color and spacing (not a fan of the default bootstrap spacing classes). And then of course custom CSS for navigation, typography and other misc widgets.
But the bulk of the main layout is handled entirely with bootstrap classes. And because nearly everything can be specified with the responsive breakpoint prefixes, I don't have to write repetitive media queries.