I have opinions on this that will keep us here for more than two hours!
Firstly, I want to make a rather pedantic point that Bootstrap, Foundation, etc. are not frameworks. They’re toolkits. That’s not a criticism, but it’s important to note that, where a framework offers guidance and standards, toolkits like these go a step further and offer bits of styled/opinionated UI.
The comparison I make is that Symfony is a PHP framework, whereas WordPress is a blogging platform written in PHP. The difference there is the difference between a framework in its truest sense, and a UI toolkit like Bootstrap, Foundation, etc.
Do you advocate the use of CSS frontend frameworks like Bootstrap, Foundation, etc.?
If you need a UI out of the box (e,g. internal dashboards, reports, etc.), definitely. If you have your own design to implement, use a true framework, or start from scratch.
Have you used them?
I have, but via clients who’d realised that they weren’t the right thing to have used.
For more opinions like these: https://www.youtube.com/watch?v=xR6V9bHlrx0