I currently have a Professional and two Industrial Mousetracks and been using Itac trackballs for years. The ball bearings are noisy compared to, say, a Kensington Expert. These are professional quality - the company even offers a refurb/rebuild service. You can see inside one
here.
They are a three button pointing device & that's it - no scrolling. The small button near the pad is a speed toggle. Basically it shifts the DPI to much lower to enable fine movement. I find the Professional & Industrial very comfortable to use but trackballs are a very personal choice - you might not like them at all. The buttons on the sides are the expected left & right buttons and the long crossways button is the middle button. I just curl my thumb under to hit that button but I have a friend who just hates the placement of that button and won't use one of these.
If you're shopping on Ebay be careful: Itac have made many models over the years and some won't work on a modern computer or are programmed especially for one company or other with special functions - no middle button, for example. Some are for Sun computers and won't work at all for you. If you pick one out on Ebay, first just ask me here about it before you buy.
