After running it for about two hours, I have experienced nothing that makes me think this is anything but the final version. It is absolutely ready for prime time. I have no fear about running it on my production machine. Sometimes beta software feels like beta software. This feels like polished, retail software.
Yes, it is quite snappy. I can't find anything in the interface that seems sluggish. I am running it on a stock, 15" MBP from the previous generation. I only have 2 GB of ram. It feels like I got a ram upgrade. I have been looking at the activity monitor to check my CPU usage. The most I have been able to push it to is about half running and intense Logic Pro file. Watching a 1080 P movie does not even cause it to break a sweat.
Speaking of movie playback, QT is what it should have been all along. It is light, clean, and useful. The bezel is completely gone. That, alone, is almost worth the price of admission. When you click on the green window button, the window actually resizes to fit the screen as it should have all along. Additionally, you can now do video capture right from QT. It works flawlessly. There will be a new era in how-to videos for the Mac. You can also make video and audio recordings from within QT. It is a lot easier than having to open iMovie or GB. Again, the features work flawlessly.
The enhancements to the Dock are subtile but meaningful. Stacks now works as it should. You can drill down into folders without leaving it. Previewing all windows in a running program, even minimized windows, is now easier than ever. The feature works a little like it does in W7 only better.
The install was painless. It was completely unsupervised. Other than the Welcome screen, I would not have known that had installed anything new. Everything looked exactly the same as before, including my wallpaper. The first thing I did was open system profiler and check the OS version number. Do not expect a whiz bang new look. The refinements are subtle, but real.
I am no developer and will not be able to answer the types of questions that advanced users will no doubt have. Still, if you have any questions within my power to answer, I will be more than glad to. I mostly started this thread because I have been looking for this type of first hand review and discussion. Well, here it is. Have at it.