I use it for professional work where I know I need a lot ready-to-go stuff baked in like date-pickers and such. Not so easy-to go with custom styling though.
Eduardo said shouldn't waste my time with ES5 so I try not to. ;)
I used it for some private stuff lately where I only needed a grid and was looking for the leanest framework there is.
I use it for a lot of professional work where I might need more than just a responsive grid. Has a great set of mixins and components and also some nice JS-modules. I love that its so style-agnostic. Really easy to add custom styling.
I use it to manage my Repos for private projects. I love the clear UI and for me it is way easier to manage than in the shell.