UPDATE 8/1/12: Quicktime in Mountain Lion now supports AVCHD, great for Sony and Panasonic users out there!
Took the plunge and can report that wifi has not been a problem. Updating took 20 minutes and was simple. Redeeming the update from the App Store was fun. You have to wait for an email with a code to open a pdf with a new code that needs to be entered for redemption.
Everything is much faster, from scrolling and zooming through Safari and general internet speed has been quicker than Lion. I haven’t seen a spinning beachball yet, not even in Final Cut! The new Safari + Mountain Lion offers in window tab scrolling which I do find myself using. The dock is isn’t transparent anymore which looks modern in comparison. If you have an Apple TV then airplay mirroring works as flawlessly as it does with an iPad.
Apple doesn’t allow you to run apps downloaded outside of the App Store by default. You have to go to system preferences, security & privacy, and select anywhere under the general tab. There is already an SMC update for Retina users that improves stability issues from sleep/wake.
Finally, the fake stitching is gone from the calendar but the faux leather and digital torn pages remain. Progress not perfection.