I personally do not "write" responsive. I write desktop, and I write mobile. responsive is for lazy people!

Bootstrap allows you to create responsive sites faster using predefined classes. The downside is linking to thousands of lines of code that create the so-called bloat, which means that making changes to the styles becomes more difficult. It also takes time mastering Bootstrap - familiarizing with the grid system and the predefined classes.

Responsive website:- Have same css just different styling that work on conditions that mean width & height size.

Why use bootsrap when you can add your own
@media only screen and (max-width: xxxpx){ ....... }
use width in % for some class along with inline-blocks and there you have it

Responsive is awesome and the design is modern and sleek for the current era of time we are in. Its the future for the time being.

To answer the question.. bootstrap is awesome.. saves so much time and gets the job done 'roughly' the same way.

grid frame work (960 grid) is the best alternative for bootstrap.