960gs is just a grid system.

Bootstrap has a similar grid, but it also has nice styles for everything else.

Bootstrap is not targeted at someone with design skills or someone with access to someone with design skills. It's for programmers like me who don't have artistic ability who are trying to launch minimum viable products that still need to look professional.

I had originally started by trying to customize templates from themeforest, so my sites would look professional except for the parts I had to customize. This isn't good enough.

Bootstrap enables me to build professional looking websites from the ground up (Admittedly, my sites probably look very similar to many other bootstrap sites, but hopefully, they'll work on theming next).

It's much more than just a grid. It has patterns for buttons, modals, pop-overs, hero-units, forms and such (including a small unobtrusive JavaScript library). If you need to rapidly prototype designs and layouts, use it. We use it as almost a living wire-frame. Where we can see interaction and UI/UX very close to the full-design spec with little effort from a designer.

The grid is only one of its features. I get the most utility out of Bootstrap's prebaked design features that I have to custom code every time I start new projects. From warning/error/success messages to popovers to simple form aesthetics.

When the most primitive iteration of my application already looks great during development when it's just a simple CRUD prototype, I no longer waste mental energy by being distracted by things not lining up, lacking padding, and generally being no-css-yet ugly off the bat.