I'm a designer and recently started a side project with a friend. We decided to use bootstrap because it was super easy to get started with off the ground..no reset stylesheets, defining default fonts, sizes, etc, and no implementing a grid framework. It was all right there. And for the most part everything was generally a good starting point. I changed the header color, used a different font for the logo (a google web font), and then built off the base that bootstrap gave me.
I say it's useful for two things: quick and dirty way of getting a prototype barely functioning and looking decent. The second is getting me right into the design cockpit without worrying about all the defaults and resets and foundation of it all. I change a few heights/widths/colors/fonts and we have something decently unique.
Edit: I might add that if a.) my app took off or b.) I was presenting it to someone important, I would make sure it was very unique and all my design and UX principles were put into play. But for simply prototyping an idea, or having a good base for a project, it's a great tool.
I say it's useful for two things: quick and dirty way of getting a prototype barely functioning and looking decent. The second is getting me right into the design cockpit without worrying about all the defaults and resets and foundation of it all. I change a few heights/widths/colors/fonts and we have something decently unique.
Edit: I might add that if a.) my app took off or b.) I was presenting it to someone important, I would make sure it was very unique and all my design and UX principles were put into play. But for simply prototyping an idea, or having a good base for a project, it's a great tool.