This week I wrapped up my first Bootstrap 3 application for testing. Up until this point, I was using version 2.3.2 due to IE7 compatibility requirements.
The transition wasn't very difficult since most of the components are the same with fancy new names, so most of the learning curve just involved having the Bootstrap documentation pulled up on monitor #2 to grab the new class names while still using the same technique from Bootstrap 2.3.2 apps.
Since Bootstrap 3 is responsive by default, it's pretty much essential to import respond.js now to get media queries to work in IE7 & IE8, which is no biggie since I was already doing that to give my Bootstrap 2.3.2 apps semi-responsive layouts.
I was never a fan of the older Bootstrap's way of trying to make everything as shiny and gradient-like as possible. While it's still possible to theme Bootstrap 3 that way, it's turned off by default which means adding just a few custom lines of CSS to dumb down the padding and border-radius allows me create some pretty business-ish applications without an incredible amount of effort.
In fact, the amount of cross-browser compatibility and just the overall amount of extended framework features that shipped with Bootstrap 3 almost makes building the interface boring.
I have no intention of converting any of my existing applications to Bootstrap 3 from the older versions until I have a very good reason to do so, but moving forward I'm definitely going to be building new apps using Bootstrap 3.