One of the lesser known features of Android 4.4 was a new runtime called ART, which should eventually replace the Dalvik runtime. The folks over at Android Police have a thorough comparison examining its effects on battery life and performance.
For the uninitiated, ART is the new runtime that executes application instructions. In other words, when you run an app, it goes through a runtime. Previously, Android's runtime was called Dalvik and, while it did OK, it was still a bottleneck as it only ran code at the moment it needed to (what's called a "Just-In-Time" or JIT compiler). ART, on the other hand, uses an Ahead-of-Time compiler (AOT) to process application instructions before they're needed so things run more smoothly for the user. A lot of people have turned it on in 4.4 and reported great increases in performance and Battery life. The folks at Android Police did some tests to see how accurate these claims were.
Android Police found that there's still plenty of optimization left to be done. At the moment, ART can squeeze out very slightly better battery life and performance than Dalvik can, but you probably won't see massive gains -- and those who claim they do are likely seeing a bit of placebo effect. Furthermore, while lots of apps have been optimised to work with ART, some don't at all -- especially things like Xposed and GravityBox, which offer a ton in the way of tweaks. If you want to use them, you have to switch back to Dalvik.
Keep in mind that ART is still new and hasn't been optimised for many devices or apps yet. If you want to try it out for yourself, users running 4.4 can enable ART in the developer options on their handsets. For a much, much more thorough rundown of the differences, check out Android Police's benchmarks at the links below.