My computer is a 13" 2012 cMBP (see sig) and for the most part, it is perfectly smooth. There are some minor areas where there is a bit of lag, but that is fine. Things start getting bad when I plug in my external monitor. When I do that, the lag gets about three times worse when I do things like open mission control or launchpad, or opening and closing tabs in safari. It is almost like my computer is having a hard time driving that display. Boot times are a little bit slower, as well as shutdowns, but I don't do either of those very often at all. Oh, and I have actually gained about an hour of battery life after installing yosemite. With about 75% screen brightness and only surfing the web with iTunes and Messages opened in the background, I am getting about 5 hours of battery life VS 4 hours on Mavericks.