I've installed Oneiric from scratch (didn't upgrade), and I'm having troubles with desktop effects: Unity is very slow - whenever I click on the workspaces icon, I either get no animation, or the animation just hangs for a second or so before it's completed. Other behaviors feel similarly unresponsive. For example, whenever I try to move a window on the desktop, it will lag behind for a while before it catches up with the mouse pointer. Gnome Shell feels slightly better, but not by much.
The CPU is an AMD Athlon II X2, I have 4 GB RAM, and the video card is is an Nvidia GeForce 6150SE nForce 430.
I've tried playing around with nvidia-settings; I've installed/removed every single Nvidia driver in the repositories (currently using nvidia-173); I've changed the Compiz settings following solutions for similar issues (unchecked "Detect refresh rate", and so on), without any success - in fact, I didn't even manage to make things worse. ;)
glxgears looks great, and it feels like the driver is working really well for everything except desktop effects. Unity 2D is fine, and I'm using it right now.
nouveau
driver to see if you get better support there?