Very pleasant
This is my first experience with a trackball and I'm pretty pleased. The ball glides smoothly and noiselessly, the ring is even easier to manipulate than a scroll wheel, and the buttons are easy to access - a real treat.
There are two minor drawbacks, however: firstly, I find it harder to make precise selections than with a mouse, and secondly, the lack of a third button puts me at a disadvantage (I use it a lot for copying and pasting).