I have an older computer, an early 2008 iMac 2.8 Core 2 Duo, maxed out at 4gb ram. I was running 10.8.5, and things were pretty slow, as you'd expect from an old computer. I'm still trying to keep things current, though, because I'm going to buy a new iMac at the next refresh and this will make the transition easier. Back when I was running 10.8.3 I updated to Mavericks when it first came out because I'd read that the memory compression scheme would improve the performance of older machines with limited ram, but it was just the opposite.Everything was soooooo slow. Even opening a folder with a few documents inside, or a short TextEdit document brought up the beachball, and after 24 hours of this I restored 10.8.3 from a clone. Today I installed 10.9.3 over 10.8.5 using the full 5.6gb installer from the app store, and my computer is running great! It's noticeably faster than 10.8.5 and I haven't had a single issue at all except for a few incompatible plugins. I've only been testing it for a few hours, but everything works and it feels like my computer got a couple of years younger. Anyway, I just thought a positive experience might convince some of you with older Macs to try it.