It only took about 9 hours to install. Granted my MacBook Pro is 5 years old, and has a slow, 350 GB hard drive, but it does have 8 gigs of ram and that helps when running apps. Anyway, although my mac now runs a little slower and uses slightly more memory, I like it. I particularly like the new, flat looking user interface and the dark themed menu bar. And so far, of the few applications I've tested (Kindle for Mac, Curio 8, Xcode 6, Crosswords, Moonlight Mahjong and Full Deck Solitaire), all seem to work perfectly. But since I use it mainly as a development machine, I still have some things to test before I can be completely satisfied.