What I think is there's something wrong with your Java installation. Remove it and install again from the Sun website. Also try both the standalone and browser version of Minecraft.
The performance improvements have made a big difference to me. Instead of around 60, I now get around 100-150fps. I think it's mainly occlusion culling that doesn't render what's not visible. That uses your CPU so if your CPU isn't very powerful, it might even give you worse performance. Switching off Advanced OpenGL probably disables that.
What's also nice is that the GUI is now scalable. That should first of all fix the problems of the mouse area being smaller than the GUI preventing you from clicking half the screen (at least in Full Screen mode as far as I noticed), but also make the GUI less intrusive, which is nice. Minecraft looks huge now for that reason.
It doesn't seem the pistons are in yet, maybe 1.6.
It would of been nice if that was in the changelog... i'd built a store room, so had an inventory full of stones, no tools, and couldn't get out of my (very underground) store room as the ladder was broken.