Bootstrap is a really good product but you should not forget what is the real purpose of bootstrap: making a website quickly.
Because of this Bootstrap is not what you should use when you want to create the website for your Startup, it is something you should use at an hackathon or to put together an MVP. In both cases if the development should go any further Bootstrap should be dropped in favour of something more flexible and personalizable, other way you will end with a website that looks like too many others, with low personalization possibilities and that is now too advanced to modify easily.
A good alternative might be Zurb Foundation http://foundation.zurb.com
Or better, if it's just a grid you need,which is in the end the most important thing in Bootstrap that does not need personalization, look into something made specifically for that:
Or best solution of all, create something unique for you, so that you might learn something new, and you will also know you product better.
Bootstrap & c. are great but do not let them make you lazy and take all the fun out of creating something unique!