In addition to what Hristo said I would like to add
In the process of developing Nop-Templates.com
themes we had a lot of discussions about Bootstrap, but in the end, we decided to dismiss it as a possible approach and if you are interested I can explain the reasons for that. 1. It does not follow the best practices.
One of the major issues I have with Twitter Bootstrap is that you end up with a whole lot of DOM elements crammed full of classes. This breaks one of the fundamental rules of good web design, the HTML is no longer semantic and the presentation is no longer separate from the content 2. The Responsive version of the Website is not optimal.
Actually, Bootstrap does not give you a truly responsive design. Bootstrap gives you 4 break points and almost nothing more. The idea of the responsive design is not that you can shrink the content of your website so that it fits on a smaller device. The idea is to build a layout based on the purpose of the website, that allows users to access the information as easily as they would have if they were browsing from a desktop machine. 3. Your Design in Limited.
The 12 column layout with 4 breakpoints is a huge restriction on the possibilities of your design. Having a predefined frame that you are not allowed to go out of can result in similar and boring looks of your website. 4. It is Lazy, not better.
If you are capable of writing good CSS you can achieve everything that Bootstrap gives you and beyond that with not a lot of work and you will get rid of the conflicts that you can experience when using class-based grid systems. 5. You will not be taken seriously.
Bootstrap is often considered a tool that is used by people who have limited knowledge of front-end. So if you decide to use it on your project it is very likely that you will not be taken seriously by someone that is familiar with that matter.
There is more that can be discussed about that, but I believe that in that five points I pointed out the biggest disadvantages of Bootstrap. Of course, this is only my opinion on the matter and if anyone else has something to add I will be happy to read it.