Not as badly as you might think! Playing around with the new box model on a test site and on the bootstrap site (via firebug), I only found four noticeable changes in appearance:
- The search form was noticeably shorter and more cramped. I fixed this by adding height: 100% to the interior text input element
- Text inputs are less wide than they should be when you use bootstrap's span classes
- The navbar's vertical and horizontal dividers disappeared. I fixed this by adding 1px of padding to either the side or bottom of the divider, as the case required.
I'm eager to use it, but as I use Bootstrap I imagine it breaks it ?