I dont have any experience with the track ball on the 9000 its a little newer than the older curves. When I had a trackball device I bought a 10-pack on ebay and switched them almost monthly.
How difficult is it to change the trackball? i have the 8330 curve
On the older models up to the Bold 9000 you can pop it out yourself. With the 9000 you can't pop it out, the whole section around the trackball has to be taken out professionally.