No, really, it isn't. It just teaches you to fill up your html with a lot of useless classes while teaching you nothing about css, unless you want to change how it looks, in which case you have to basically rewrite everything.

It may be a good substitute for a first draft, but your website will be much leaner and cleaner without it.

Also, I can never get over the fact, that the bootstrap devs seem incapable of delivering UI elements like dropdowns without additional javascript or can't even include a no-js fallback. That doesn't really create confidence in their abilities as web developers.

My experience is that Bootstrap makes 95% easier but the 5% that doesn't cooperate eats up all the savings of the 95%. When it doesn't do what you intend, it's a black box that usually ends up being "fixed" using obscure CSS or JavaScript fudges found on Stackoverflow etc. It feels like: "Just put in 'grf:snig;bleppo(){99.foo()}' here, and that should fix it."