There is an inherent usability in the familiar and mundane, but some web designers will feel Bootstrap's sense of style is a little too familiar, and a little too mundane. These designers would rather start from scratch than start building another web site that looks "Bootstrappy".
One thing to keep in mind is that Bootstrap is not a "theme". Bootstrap is a framework. If you accept all of the defaults you'll build yet another Bootstrappy site. It takes a little bit of work to make a unique looking site, but building a unique look is certainly possible (take a look at Built With Bootstrap for inspiration). There are also now dozens of sites like Bootswatch (motto: saving the web from default Bootstrap) where you can download colorful themes for Bootstrap. There are also some limitations and quirks to Bootstrap, but like any framework if you follow the conventions and direction the framework wants you to follow, you'll have an easier time.
There are more reasons to like Bootstrap than just the three reasons I provided above. Ultimately, though, I like using Bootstrap because someone carefully designed a framework that is both re-usable, extensible, and provides sensible defaults. If you haven't used a CSS framework before, 2013 would be a year to try one. If nothing else, you can learn how to better organize and design your own presentation rules.
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