Generally you will hear people say that it makes websites look exactly the same. This is not entirely accurate since there are CSS preprocessor versions of Bootstrap (LESS or Sass) and they make it really easy to override the defaults so that your site can look unique. In fact you might be using websites that implement Bootstrap without you even noticing.
It is also worth noting that responsive design is now part of the Google search algorithm, and Bootstrap 3 is all about making your designs responsive. Bootstrap is easy to use and helps to make responsive designs that look good on all devices, from smartphones to large desktops.
Basically with some Sass or LESS and custom CSS you can easily shape Bootstrap around the edges to make it suit all your visual needs.