When ever a new version of OS X is released, Ars Technica puts out as thorough a review as anyone. This release is no different: OS X 10.11 El Capitan: The Ars Technica Review

A few highlights:

• New system font: San Francisco
• Improved Mission Control.
• System Integrity Protection. Means apps like TotalFinder won’t work without a tweak. Bummer.
• Responsive Design mode in Safari. Cool for us developers!

So do they recommend you upgrade? Most certainly, but this quote sums up the differences between El Capitan and Yosemite:

Almost all of El Capitan’s updates are aimed at detail-oriented power users who are intimately familiar with the platform and its apps.