I have a dual core i5 MBP that I've done upgrades to in an effort to keep it speedy such as add RAM to the max of 8G, adding a 256G SSD + 1TB to make a fusion drive. Each were improvements. By far the best thing I did was put Yosemite on it. I was considering getting a new rMBP but am blown away at the performance boost I just got. And, I did it as an upgrade over Mavericks so the boost I'm seeing is not what you get when you do a fresh install. Just wanted to let others know that may be on the fence - go for it.