To me, one of the most annoying parts of prototyping a new web application is getting a presentable look and feel up quickly. When I'm prototyping stuff I love to have a good looking interface from the start, even if that interface will be changed before going live.
As for the elements it offers, some of my favourites are:
- All typical HTML elements are styled, including decent typography
- A top bar to be used for menus and such, with drop-down menus to be activated easily with JS
- Ability to pretty-print code when used in conjunction with google-code-prettify
- Labels and buttons, with styles for success, warning, notice, etc
- Alert boxes, with corresponding styles, great for showing form submission status etc
Bootstrap is built on top of the LESS CSS preprocessor for ease of extending and integrating into your apps. Personally I found that the precompiled stylesheet was more than enough to begin with however, and I'm more of a fan of the SASS preprocessor anyway. The JS modules are also very easy to integrate with very little code needed to hook them up to their appropriate classes.
I jumped into Bootstrap shortly after its release, and have been using it ever since for my internal prototypes and eventual applications, including an application in the process of being deployed. Bootstrap played an integral part in getting that project complete under it's tight deadline.
Highly recommended to all!