I have been waiting a long time for a good themable template system that I could integrate once and support easy color changes.
So I was very excited when the jQuery-ui project started consolidating their API and using a common theme system. I fully integrated it while making the transition to html 5 and started waiting for the many new features to graduate out of their development version, and waiting, and waiting..
Every time I would go look for a new jQuery-ui version I would wind up looking at bootstrap, which sounded promising but laced a set of pre-defined themes.
So we took the plunge and integrated bootstrap to handle our site header/footer layouts. Unfortunately this was done right before version 3.0 came out so we had to quickly learn what combination of layout tags worked all over again.
Overall I would have to say it was fully worth it, we got some nice layout components for our client websites with a few jQuery components mixed in, and pretty much picked up all the responsive design features.
Since then I have incorporated it into several websites, including my Corporate website with great success.