It depends on what you want to optimize for. If you're starting a startup and you anticipate having a dedicated design team, I think you're better off making your markup match your domain and hand-rolling the CSS instead of using a framework like Bootstrap. This is the ideal approach IMO because it lets you much more effectively keep the styling out of the markup and in the stylesheets. CSS frameworks by definition require you to put styling in your markup.
But if you're just trying to bang out a small project quickly and have it look nice without needing to muck with CSS too much, then a framework can be very useful. These days I prefer Semantic UI over Bootstrap: