For anybody holding back, this update is the real deal. Snow Leopard was incredibly fast and incredibly stable. The transition to Mountain Lion brought a lot to the table feature-wise (mostly surrounding integration with iOS and its devices), at great expense from a performance standpoint. When I "upgraded" to Mountain Lion it felt like my machine aged 2 years overnight (while getting some great features at least). Mavericks feels like it gave me those two years back in just as short a timeframe. Completely unscientifically speaking, everything is just snappier. Time to open apps, transitions, etc. Couple that with extending the battery life of any machine that loads it, and it's looking to be the best iteration since SL. I'll take an "under the hood" update over a feature-centric one any day.