Well I was one of a million plus who downloaded Lion (OS X 10.7) the latest version of the Mac OS the first day it was offered. It was a first in many ways. The first OS to be downloaded from the new App Store. The OS installs itself and reboots your system for you. It has many refinements. It is a closer match to iOS with the use of gestures on the trackpad.
This version won’t run on hardware older than my August 2008 iMac. It will not run old PowerPC apps that once ran under Rosetta. While it has a number of modern UI features, you can change some back to the old style - like the scroll direction, whether you use full screen or not, etc. It took me just two hours to download and install.
Very few problems so far. Magic Prefs tangled with my touchpad. My dot mac email wouldn’t work until I reset its password. My MobileMe remote disk disappeared from the desktop until I changed the prefs to add a local disk - the content of the local disk I had before was saved for me in a folder on my desktop. And Safari dropped my ClickToFlash plug-in. The replacement was an updated version which I tracked down. The old Quicken doesn’t work but Quicken Express, which I use, is fine. The Adobe programs work (CS5) as far as I have tested. I am pleased with how smooth it installed and once again it scoots along fast on this 3 year old system of mine.