I'm using aixgl and direct rendering works in compiz fusion.... PS plays just fine on my PC, no noticeable lag except recently with the lighter2 slowness bug. And as for usability I find having it on is very good. The ability to fling my mouse to one spot, see all the different tasks on that virtual desktop, click have it all rearrange nicely really improves my efficiency when multitasking. I also tend to forget which desktop I left a particular program on, and the little preview isn't always enough so the desktop cube is a fun way of not having to cycle through slowly, I can just hit a button and fling the cube around until I see what I need. Its really not that much faster but its nicer to look at

I use the fire plugin to leave notes for my wife on my screen when I'm gone, the fire letters are hard to miss and better than a note on the fridge. Wobbly windows are just ridiculous though

. The drop shadows on the other hand are not as they give a depth feeling for what order the windows on my desktop are in which is handy when I'm using multiple similar screens and am thinking back to which one based on the last time I had it with focus. I use transparency in firefox and a text editor when doing coding tutorials since I don't have a multi monitor setup. This is all positive from a usability standpoint.