Does anyone know if the image stabilizer affects the image quality in any way?

I beleive the image stabilizer will cause the auto-focus function to "breathe". In other words, focus will oscillate back and forth trying to hold focus as the stabilizer prism operates. It's best not to use both simultaneously. Also, never use image stabilization when you're mounted on a tripod and simply doing pans. The stabilizer interprets a slow pan as something to try to stabilize and you'll get a series of jerky pans.

Is that second part definitely correct? I thought that Canon's optical stabilizers essentially worked like dampers: You get a slight lag at the start of a smooth pan, but after that, the stabilizer should remove any jerkiness, rather than introduce it.

I have seen first hand how IS with the cam mounted to a tripod is NOT good. We did a shot a week ago and I forgot to turn the IS off. After reviewing some of the footage, some of the pans did indeed have a little jerk to them. I had to use SteadyMove to try to remove that.

I have noticed that if the IS is on and tripod mounted during a pan it is quite smooth however when you stop the IS doesnt and if the subject is moving it gets jerky.But I think a part of that problem is due to operator not realizing that the IS is reframing beyond what was intended and the OP moves the camera back.Thus begins a bad cycle.Normally would not use IS while mounted but I have done some testing.Also can occur handheld if your support is very stable.(sitting in a chair)

The IS tries to fight any camera movement. The problem arises when that movement is intentional. Therefore, turn off IS when the camera is mounted on a tripod.