Remember: Twitter Bootstrap's UI is just some default styles. I think Bootstrap is a great foundation, just like 960gs and Blueprint. But it's just that: a foundation. Overload or add your own custom styles, and you can easily break away from the default look and feel.

In fact, just yesterday I wrote a blog post about my thoughts on using Bootstrap in "production": http://www.projectorpm.com/blog/2012/03/10/dont-hate-twitter... (FYI, the blog and the product are both built using Bootstrap)