To Bootstrap or Not to Bootstrap ...

Once you know CSS, HTML, and JS well enough, do you find it easier to forgo CSS libraries such as Bootstrap? What size of a project do you feel such libraries become necessary or a hindrance?

Personally, at first I thought, "Oh hey, this Bootstrap is convenient," but then I began to use it and based on certain specifications and the fact that I already sort of know how to create a responsive design, it seems less useful than I initially thought.

What are your thoughts? Can you recommend better CSS libraries?