I am facing very sluggish performance in my MacBook Pro after I upgraded it to Yosemite. Mine is 8GB, i7 mid 2012 system.
Usually I find my unused RAM <25MB even if I work in terminal, say around 8 Safari tabs open (excluded fb) and some light-weight programming editor open.
I remember I never faced such sluggish performance in Mavericks or Mountain Lion. I have closed all unwanted graphical effects, but still no improvement on performance.
Below is the screenshot of my activity monitor:
Please note my Preview was closed while I took this shot; which is in a way super weird.