Twitter's Bootstrap CSS framework has been wildly successful. I am using it on some (behind-the-firewall, unfortunately) consulting projects and am very happy with it.
Like anything else successful in tech, it has its detractors, but I'm truly surprised by the lack of substantial criticism. The biggest complaint, which I've made myself and have cautioned clients about, is that if you're not careful your site will end up looking too much like all the other Bootstrap sites that are out there. This problem occurs if you don't take the time to customize Bootstrap -- something that is relatively easy to do with a little tinkering with LESS.
In Why Bootstrap might be very important Dave Winer argues that this apparent drawback is actually a good thing and draws a comparison between Bootstrap and the original Macintosh Toolbox. In short, he's reminding us that UI standardization is beneficial. Read the whole (short) thing and skim the Hacker News discussion it spawned and draw your own conclusions.
Another valid complaint is that sites that use Bootstrap don't completely separate content from presentation in part because key Bootstrap CSS classnames describe layout, not content. In practice this is only a minor issue and is probably unavoidable with a CSS framework, but it might upset a Semantic Web pilkunnussija.
Bootstrap (and LESS) have changed my perspective on front-end development. I'd like to apply some of the lessons learned to the iUI Framework, but that's a subject for another time.