Android's Dalvik runtime was "just fine" in terms of performance, but it was still a bottleneck as it only ran at the moment assets were needed (what is called the "Just-In-Time" or JIT compiler). ART instead uses an Ahead-of-Time compiler (AOT) to process application code before it is needed. Some users have turned it on in 4.4 and report increased performance and battery life. But what are the actual numbers?
Android Police put this to the test 1 2 and ran actual benchmarks to put the claims to the test (6 benchmarking apps and 2 browser benchmarks using Chrome). While there are a few gains to be made, none of them make it really worth switching over at the time of this writing. It almost null effect on battery life and the numbers even "favor Dalvik by a little bit". ART also breaks some apps (like Xposed) which has prevented some people from even enabling it. The author even noted that "it looks like gamers would be better served by sticking to Dalvik, for now."
You probably aren't benefiting from ART just yet, at least not fully. Android L will have ART as its only runtime and will be fully polished and completely integrated. Developers will then follow suit at create/update their apps to be compatible and optimized with ART.