Mavericks working great on an early 2008 Pro, using ATI HD 5770 GPU, 8 gig RAM, four SSD hard drives, and two displays. 10.9 is def. better at handling multiple displays. Wiped my boot drive and did a clean install by making a bootable USB image of the Mavericks install package, which you can get from the App Store. There's multiple ways to do it, here is one way using Disk Util (my preferred method). Longest part of install was backing up data, contacts, photos, iTunes library, etc. etc. Actual install, under 25 mins. Data migration back to new Mavericks install was seamless. One thing I noticed, sound is improved, at least in iTunes. Running HK sticks and subwoofer via the line out. Maybe it's just me, or maybe Apple improved it, either sound in Mavericks or in the new iTunes. Curious if anyone else notices cleaner sound, better volume, better base on their Mac Pro.