I have used Kengsington's trackballs for my music recording as the manner in which I used to record required some rather long scrolling at times. I have actually changed the manner in which I approach things but I still use the trackball as I have become a big fan of this input device. I am online for my work and I like the trackball as you can custom program each of the four buttons and the scroll ring is great for long pages. I use the Kensingon Expert mouse (actually a trackball) at work now and highly recommend this input device for both work and creative stuff.
I am quite surprised that there aren't more trackballs around. I feel the reason might be the price as trackballs are quite expensive compared to a $12 mouse and they are to very portable.... Furthermore, I went through a phase of trying out other company's offerings while on the hunt for a good trackball and I can share that the other trackballs by Logitech and Microsoft don't come close for serious inputting work. Perhaps for certain games the smaller trackballs work and I know of some people who like to user their thumbs etc. Everyone has to find their own way and input devices are very personal things but from my experience, the Kensington products are in a league all of their own right now.