At least. Given how iOS 8 has so far very annoyingly slowed down my iPad2 and iPhone 4s (yes, I'm an idiot for installing it), I'm going to wait awhile for others to sport on Yosemite's effect on older hardware such as my 2007/8 iMac 7,1, which pretty much flies with Mavericks and the SSD I installed.
Why don't you make a partition and try it out for yourself. If you don't like it then simply delete the partition. This is what i am going to do.