I Love the Kensington Expert Mouse
I believe I bought my first computer in 1994, though I had been using my Dad's computers for years and years before that. It came with a mouse. Shortly thereafter I replaced it with a trackball named (oddly enough) the Expert Mouse, by Kensington. I've used a handful of different trackballs as well as several versions of the Expert Mouse. For long-term use, I doubt I'll ever use anything else.
I primarily like trackballs better than mice because they're stationary: at the end of the day my wrist and arm aren't so tired. To me, it just feels like a more comfortable way to work.
What first interested me in the Expert Mouse was the size of it. It's huge! The four buttons are very large and easy to hit no matter how you position your hand. The mouse ball is also very large (about two inches in diameter) so it's very easy and fast to roll. The current version of the Expert Mouse uses optical tracking and has a scroll wheel surrounding the ball that have really refined the product from the first version I used.
The large ball makes a big difference compared to trackballs with small ones. In the 90s there were a lot of manufacturer's building trackballs and the majority of them used a tiny ball to track with, where your thumb was used to roll the ball; moving across the screen would require several swipes to move. With the large ball of the Expert Mouse your palm or fingers can roll across the ball and take you all the way across the screen. It's simply a matter of efficiency. Logitech appears to be one of the only other popular manufacturer's who still create trackballs, and they have increased the size of the ball, however it's still not as large as Kensington's.
Like just about every piece of hardware, the Expert Mouse has software to go with it (MouseWorks, in this case). You can, of course, program things like the buttons, scroll speed, and scroll wheel behavior. The software holds a great deal of flexibility in the button click configuration with chording (click the top or bottom buttons together to perform another action, therefore giving you six programmable buttons), and set the buttons to perform normal types of clicks (left, right, double-, etc) as well as special events such as cut/copy/paste, create a popup menu, display a menu of running applications, launch a URL or open a file, or even any keystroke combination--and all of this can be set per-application, if you choose. Another great option is the mouse speed control: you have fine tuning control over the pointer response to quick and slow movement, so the Expert Mouse can better be tuned to the individual. I believe this software works with any Kensington mouse/trackball, and it really is a nice program.
One of the things I have come to appreciate most about Kensginton is their Warranty service--it is phenomenal. The Expert Mouse comes with a five year warranty. Over roughly the past 14 years I've purchased four Expert Mice for home and work. I've used their warranty service twice to get 100% free replacements. The customer service was very fast, responding within just a few hours of when I submitted a support ticket, quickly verifying the problems, getting my address, and shipping a replacement. I've used a few other Kensington products and they're all high-quality, but it's thanks to customer service like this that I will stay a loyal customer.