Of course, I've heard about the 2D performance problems of Nvidia cards when applications use Xrender. I didn't pay much attention as I thought that this problem wouldn't affect me—I just have a 7300GT and not one of the cards of the 8000 or 9000 series for which these perfomance issues were reported. Alas, I was wrong...though I notice a performance hit only for firefox and KDE4 applications. For the latter, however, the slowdown is truly unbearable when compared to the open nv driver. See also the update of my KDE 4 rant here.