Bootstrap is a very useful front-end component library for faster and easier web development. You won't have to worry about having a decent user interface when you use it.
I love Bootstrap because it solves a problem that I have. I'm not good with designing web app user interfaces. That is why I made this Bootstrap tutorial for beginners.
As a proof, you can see that I'm using Bootstrap for my online tools site. We are using Bootstrap in many of our tutorials as well. One example is our PHP, MySQL and OOP CRUD Tutorial – Step By Step Guide!
Here are some good looking websites or web apps built with the help of Bootstrap: http://expo.getbootstrap.com
Just a little history, Bootstrap was created in Twitter. It was called Twitter Bootstrap to streamline their development. Thanks to these people, the library is still in active development.