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.
It's nice to download a new version of any software and find that it gives you not only more features, but a performance and battery bump. Definitely not the way things normally go. Usually it's just more bloat. I downloaded iOS 7 on my iPhone 4 and it turned it in to a pile of poop. Not a huge deal as I'm grabbing a new phone in a week, but they really pushed it by "supporting" that device. They were kind of screwed though regarding how far back they could support old devices with iOS 7 being such a major overhaul and riding on the back of the largest processor performance jump in years (the A7 chip in the new phones and tablets). Speaking of tablets, T-Mobile is doing a crazy thing. They will be carrying the new iPads, and if you buy one with a 3G antenna from them, they have a 200MB/month data plan absolutely free, and it is for as long as you own that iPad forever, whether you are a T-Mobile customer or not. Freakin crazy. We had just made the decision to switch to T-Mobile because of their ridiculously competitive pricing and the fact that you aren't locked into a contract if it sucks, then they bust this out. I feel like they are the only carrier out there competing.