CSS frameworks can hurt usability and slow you down
I was faced with a tough question today. My boss wants me to redo everything I have built for the last few weeks using Twitter Bootstrap. Now I have nothing against TB as a framework, specially for teams that do not have a dedicated front-end programmer. But for teams that have one and that person is also the designer, forcing him to use a 3rd party framework can have some negative drawbacks.
Frameworks can be good to quickly prototype new features in HTML. But like someone pointed out, a framework can never solve all user-interface problems because no two sites have the same requirements and problems. New problems require new components and a new solutions. Trying to shoehorn a framework like this into your app can hurt usability, specially if you do not take the time to customize and write your own solutions on top.
If your website is highly specialized, using Twitter Bootstrap might have very little benefit. Specially if you are a front-end developer yourself. A better solution is often to copy other solutions and build your own set of custom tools and components that fit the design you are working with. This approach will give you a productivity boost as you are working with your own code. Not to mention if you are already using LESS or other compiled languages, making your own custom framework is easier then ever and if you are like me, its also a lot of fun!