I used it at my startup, but there's absolutely no reason for me to upgrade to BS3. I think bootstrap is great for giving people a quick framework to build off of, when writing up a grid system doesn't make sense. For our next major release, I think we're going to migrate away from BS entirely. I'd rather control it all in house than play catchup with their releases. For me, it's not even the UI but the ability to control exactly what we're doing.