I have a second monitor plugged into my newest model iMac. Swiping between desktops was smooth before, but now with Mountion Lion, the image on the second monitor stutters badly when swiping. Very annoying. Also, the middle-click app no longer works. With it, I used to be able to 3-finger tap on a link and it would open in a new tab. This is a major bummer for me!