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.