I like Bootstrap, but Bootstrap has a lot of overhead that many sites do not need to use.
It would all depend on the site that I was working with as to what framework I would use. There are many good frameworks out there with many different features.
One thing that needs to be pointed out for good structural content you want to keep the structure lean and mean for speed and SEO. With all the new properties that are available now in CSS, I am referring to Flexbox and CSS Grid, many times there will no longer be a need for a framework.
I have been writing a class, for next semesters web engineering course, using CSS Grid and it is absolutely amazing what I am doing with a few lines of CSS. What use to take 100's of lines of CSS in Bootstrap, even in Bootstrap 4, I am now completing in about 6 lines of code, and it is more responsive and much more stable across different browser rendering engines.
Now that said, there is still only about a 77% support world wide ratio on some of the new properties, but that is changing quickly. When I started developing and researching the class about 90 days ago, the world wide browser support was only 52%. 52% to 77% a gigantic jump in just 90 days.
