I've already made the switch.
The card component is really worth checking. Great improvements are coming, especially when it comes for utilities.
The biggest challenge is to switch your existing codebase to Sass if you were using LESS before. They have also removed a lot of things like icons (good idea) and all the JavaScript components (that's annoying); and have greatly simplified their components (navbar, card).
It's true that Bootstrap has challengers: Foundation, Bulma, Miligram. I find foundation a little hard to get started with and a less friendly default look and feel. Bulma and Miligram are more minimalists framework and can be great if you're worried about the profusion of Bootstrap components.
Yet, Bootstrap is still the framework for which you'll find developers in the blink of an eye.