Comprehensive look at OS X 10.11 El Capitan by Jason Snell:
There was a time, only a few years ago, when OS X updates were fraught with should-I-or-shouldn’t-I peril, along with a real price tag. Those days are long gone. Should you update to El Capitan? Unreservedly yes—I’ve found it to be stable, it’s free, it’ll download and install itself on your Mac with nearly no intervention, and it’ll bring with it improved security, speed, and functionality.
The days of dramatic operating-system updates are over. El Capitan is as solid as the giant granite monolith that towers over Yosemite Valley. Upgrade, and get an improved Mac. It’s really that simple.