I'm using it on a 2009 MBP and it runs a lot better than Mavericks did. And that isn't just conjecture like "Safari feels snappier" It really is performing better.
One of my biggest annoyances with Mavericks was graphical lag. Moving windows around the display, scrolling in Mail, Finder and the Messages app.
Using an external display especially crippled graphical performance. There would be very noticeable lag when moving an active application window around the display with the mouse, literally the window would be lagging behind the mouse at like 5 frames per second.
But in Yosemite, this is not the case. I'm getting a full 60 FPS dragging windows around, scrolling. It's a night and day difference.
That is the biggest change I've noticed. I have not seen any negatives to this upgrade whatsoever. It is either an improvement or the same performance depending on what your task is. I'm quite impressed as I expected it to run poorly.