Bootstrap 3 is much nicer to use than previous versions, if that's what you were using in the past. Each element is more of a barebones styling to it, which makes it easier to get what you want out of them (such as buttons). I spent a lot of time in previous versions of bootstrap just stripping out text-shadows, border-radius, gradients, and more just to get it back to a basic component.

The grid in the Boostrap 3 is pretty useful if your requirements are for a responsive layout. It really makes it pretty simple to get things going, but does pollute your html with a lot of cruft as you sometimes need to define each size within a single class attribute.

One other thing I've found is that it makes the site more maintainable, as it's easier for others to understand your CSS and layouts. It's the equivalent to being able to hire a Java programmer that knows the Spring Framework (they're plentiful, and usually know enough to not break things). The same is the case for Bootstrap. By having a common and popular framework, you have more developers that can jump right into your front-end quickly.

Also, it's easy enough to customize[1] to get only what you need. If all you need is the grids, just remove everything else. No need for the js components.

[1] http://getbootstrap.com/customize/