Quick Evaluation of Twitter Bootstrap
Boostrap offers a 940px wide canvas, separated in 12 columns. What you do with those columns is up to you: most of the time, you'll probably want to group some of them. It's easily achieved by using simple CSS classes. In fact, in Bootstrap, everything is done with selectors. For example, the following snippet offers a main column that has twice the width of its left and right sidebars:
<div> <div>Left sidebar</div> <div>Main</div> <div>Right sidebar</div> </div>
A feature worth mentioning provided is responsive design: in essence, the layout adapts itself to its screen, an important advantage considering the fragmentation of today's user agents.
ComponentsWhatever application you're developing, chances are high that you'll need some components, like buttons, menus, fields (in forms), tabs, etc. Through CSS classes, Bootstrap offers a bunch of components. For existing HTML components, it applies a visually appealing style on them. For non-existing components (such as menus and tabs), it tweaks the rendering of standard HTML.
I tried to remake some pages of More Vaadin with Bootstrap and all in all, the result in not unpleasing to the eye. In the case I can't use Vaadin, I think I would be a strong proponent of Bootstrap when creating an application from scratch.
You can find the sources of my attempt here.