I've been running Mavericks for a day now (upgrade, no clean install), and I'm really impressed. Here are some big improvements that are very noticeable: 1) Battery life. My new Macbook Air now gets over 14-15 hours on a single charge. 2) Safari. The browser absolutely dominates the competition now with it's battery saving features and perfect smoothness on all websites, many of which used to be choppy in all browsers (Buzzfeed, HuffPo, etc). Streaming videos use very little CPU power too! Using the omnibar is as fast as Chrome now, and pages don't reload when you go back to them. Basically, Safari is perfect now. 3) OS speed improvements. Everything loads faster. Boot and shutdown are much quicker all of my background apps open way, way faster after login. 4) iCal is much prettier and more functional. 5) iCloud is updating much more smoothly. 6) Time Machine backs up much faster. Basically, 10.9 has fixed almost every major complaint I had about 10.8. Some of the problems should've been fixed a long time ago, but I'll take what I can get. This is a must-have upgrade.