The fans no longer spins on high when dealing with RAW photos!! All previous OS X would spin my rMBP's fans on high, if I was using Lightroom to edit raw files, or if I have Bridge open browsing through photos. With just Lightroom open I can't even hear the fans. With Bridge, it's on low spin. This'll save the life on the hardware and battery! Nice job Apple! Not only that, like others have mentioned, but if you didn't know, every animation is BUTTER SMOOTH!! I haven't seen it this smooth since I bought it in 2012. No stutter, no lag. It's a godsend.