I am for a long time user of a Mighty Mouse (actually 2 of them. And as every user of a Mighty Mouse, I have the issues of the mouse ball being stuck and needing some cleanup.
In the past I've turned the mouse around and rolled it over some issue, which helped more or less. But just now I had the urge to finally open it and really clean it. There are many "how-to"s on the web for this.
When the mouse was open, it became very obvious why it did not really work well anymore:
opening the white cage an removing the ball (which itself was totally clean) revealed more dirt:
Cleaning the little rolls was no issue, but putting them back was somewhat harder, as the thicker black rolls are magnetic and would directly attach to a screwdriver.
After assembling everything, the mouse responds well again, so problem solved for the years to come :)