I've loved Bootstrap since 2011 - it's well established, it fulfills it's purpose and adds some extra niceties on top. Lately though; I've started to appreciate semantic HTML.

I'm not a purist, but I definitely don't want to be seeing '.grid-x' style class names in my markup. Markup is meant to explain the content, not the presentation. That's one reason.

Here's another: While it is presumably simple to just use their LESS mixins to achieve a more semantic and customized site, Bootstrap's main target market is not those who use preprocessors - their call to action is a download button for the .zip of the compiled CSS, so that developers who don't necessarily use CSS preprocessors still have access to the tools.

This gives Bootstrap a competitive edge of some sort, as more developers can easily get started using it. As a result of this decision, though - those developers have to resort to overwriting the actual CSS to customize anything, and even then, there are still traces of the Bootstrap default styles left over, making it very easy to spot a Bootstrap website. Due to the project's popularity - a lot of these websites end up looking remarkably similar to each other.

If I'm working on a huge project, I will use a different kind of CSS framework (my favourite one at the moment is http://roots.cx/axis in combination with http://jeetframework.com). If it's a quick hackathon project, a mockup/wireframe or simply just a weekend project for fun or learning, then I'd use Bootstrap.