What existing CSS frameworks have you used locally, or in production?

Bootstrap: Slow release cycle. Bootstrap 4 has been in alpha for almost 2 years. Future versions of Bootstrap should include a spinner button component, as it is widely used.
Semantic UI: Source code structure makes theme customization extremely hard to understand. Its unconventional theming system is a pain to customize. Hardcoded config path within the vendor library. Not well-designed for overriding variables unlike in Bootstrap.
Bulma: A lot of non-semantic and superfluous classes and markup required. Not backward-compatible. Upgrading versions breaks the app in subtle manners.