to chime in again, worked fine on my 2007 MacBook Pro, though the first time it seemed to do the upgrade but didn't for some reason. 2nd time it ran through and told me my HD was broken and not fixable w/o much options so I clicked install again and it worked just fine. took 30-45 min, but everything works. My Epson Artisan printer had a short hickup, I printed a photo and while it printed happily away the mac told me the printer software was installed wrong and if I want to fix it, fixed some permissions (while printing my photo...) and works fine since. LR 5.2, Photoshop CS6, OnOne photo suite works too, everything seems fine. No idea what new features I got, besides that there's now a maps module on the bottome. I use google maps, so I don't really care.
The 6 year old laptop is getting too old for my 5D3 files though, gonna get a mac mini this or next week, should be set for an other long run then.
Do backup first though! Not only with time machine, I'd recommend making a bootable backup, I use SuperDuper, other options are out there. If something goes wrong you can just boot from the external drive and restore everything to where it was.