I had searched for some first-hand experience on using Yosemite on old Macs but there seemed to be very little details around so I decided to post my first-hand experience using Yosemite on old Mac, such as my MBP 2007 2.2 Ghz. I use Macbook Pro every day constantly from 2007 to this day. Due to lack of applications support for Snow Leopard, I had to upgrade to Mavericks (a couple of years ago?). After the Maverick upgrade, I found my MBP 2007 ran much hotter than it was used to be with Snow Leopard so I had to do CPU/GPU paste (around 2 or 3 times during the use of Mavericks). But it helped very little. From what I have found out with Mavericks, the fans do not kick in even for a prolonged while after CPU went to 100 C and also when CPU goes back to idling, the fans went straight back down to idling even though the CPU was still hot. So the temperature was going down very slowly as a result of that. Did a SMC, PRAM reset etc but to no avail. That was on fresh install of Mavericks so I had no choice but to leave at that and put up with this issue every day. It was like handling a metal bodied toaster! Despite the applications opening more slowly (with HDD) but every day use was fine though. Now with Yosemite out, in 10.10.2, I decided that I have nothing to lose and installed it. My surprise was that the fan kicked in much early while the temperature of CPU was rising (started at around 70s C or so) therefore it managed to keep my MBP cooler and more comfortable to touch...and if the CPU goes back to idling, the fans do not slow down quickly until the CPU/GPU temperature settled down to their normal level. As a result of this, the MBP cooled down much quicker after some intense CPU usage so I have to say that I was pleasantly surprised! So Yosemite is doing the cooling what Maverick should have been!! It was clear to me that Apple had made the Maverick to 'preserve' the battery, to hold back the fan spinning up unless CPU was running hot for a prolonged period so that was a poor decision, especially for my old MBP. It seemed to me that it's because they claimed that the battery life is improved with Mavericks! Secondly, closing/opening the applications is quicker with Yosemite than they were with Maverick. System preferences opened almost instantly. Firefox opened and closed more quickly. Great. Even better, it wakes from the sleep with WiFi connected instantly so I can use the internet straightaway just after waking it from sleeping. It wasn't the case with Mavericks and on Mavericks, when I disabled/enabled Wifi on taskbar, there was a pause and if I re-enabled Wifi, it takes ages to re-connect to my home router but with Yosemite, it's just instant. Thirdly, visual UI, it is more customising with Yosemite than it was with Mavericks so I was able to disable Transparency completely by clicking 'Increase contrast' in Accessibility. Also disabled Notifications Center and Mission Control as I do not use them at all. So the overall experience of using Yosemite is much better than it was with Mavericks. So if you have old Macs installed with Mavericks, then you have nothing to lose and go ahead and try it. Note: I tried it first by installing Yosemite on my external HDD and I was impressed with it so I went on to install it in MBP afterward. You can try this route first but remember that external USB drive is much slower than internal HDD, however Yosemite was impressive even on USB drive. Also I did tried Yosemite beta when it first came out last year, I wasn't impressed and I discarded it after a while.