I've been using bootstrap 4 in a current project and it is actually pretty good.
Cards are way cooler than panels but don't go overboard as some bootstrap-cards themes have done where everything is a card.
Bootstrap 4 is still in alpha though.
same here, using it for internal projects where supporting old browsers is not issue, works good enough.
on a different project I used one of the bootswatch themes and last I checked those were 3.x though so still hanging there.