Bootstrap has been a valuable framework for quickly producing responsive sites. I have used Bootstrap on a number of projects now, including this site. Using Bootstrap allows me to quickly build websites that work across multiple devices. The big disadvantage of using Bootstrap is it adds a lot of styles that you may never use, causing CSS bloat. I also believe that the need for a Bootstrap style CSS Framework will disappear over the next few years been replaced with native HTML and CSS features such as flex box and CSS Grid.