Pbuffers are not a performance win – they use up extra video memory and force you to do extra MakeCurrents. Using the back buffer directly should get you better performance.
The reasons to use pbuffers have more to do with functionality than performance – independence from window size/window clipping, an extra buffer, offscreen rendering, etc.
So don’t use pbuffers unless you actually need to. (That goes for any feature.)
Thanks guys. I’ll look at the pbuffer extension specs.
anyone know if there are any plans to enable pbuffers to support greyscale buffers eg GL_LUMINANCE?
Thanks, Matt. Now I really don’t know what to do 
I’ll still have to use pbuffers to get good performance on the Radeon… Hopefully ATI will optimize copy to texture more fully.
I just tested the demo on a Voodoo 5 board. It performed horribly with render to texture turned on (below 5 fps, about 60 fps when disabled). And the reflection was totally screwed. Instead of a ball I only saw a colored bar (just as wide as the ball, with the same colors) directed towards the mountains. It looks like a one pixel wide texture stretched all over the water surface.
Well, no one’s going to fix these drivers…
As rendering to textures is getting more and more important, I wonder if someone will come up with an easier way to do it. OpenGL’s way of managing textures doesn’t seem perfect to me.
If I remember correctly Matt or Cass told us that NVIDIA is working on a easier way to render-to-texture stuff. But maybe i’m wrong and it was just a dream. Would be a nice thing though.
-Lev