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TechGearLab

The Logitech Ergo M575 Wireless Trackball Mouse is the best trackball mouse we tested. This mouse performed very well in most every metric, but especially in ergonomics. Trackball mice use a ball to move the cursor around the screen rather than requiring you to move your arm. This mouse style has gained popularity in recent years, and we think Logitech nailed the design with this one. It has some of the best button placement on any mouse we tested, too. The scroll wheel is easy to reach, especially compared to some other less traditionally shaped mice. We found the forward and back buttons are placed in a much better spot than on some mice, too. We occasionally accidentally bump these forward and back buttons, often found right next to the thumb grip spot. However, on this mouse, the forward and back buttons are on top, next to the left click button. We found this spot to be much more convenient since we aren't ever inconveniently hitting the back button when we're in the middle of an important task. 
The Ergo M575 Wireless Trackball Mouse is one of the bulkiest mice we tested here. However, this is a common tradeoff: the more ergonomic a mouse is, the less portable it is going to be. Also, we would love to see a sensitivity adjustment for the trackball built into this mouse. But you can adjust the sensitivity with your computer, so that's certainly not a deal breaker. Also, as with all alternative mouse designs, this mouse will require some getting used to. But if you're looking for a mouse that you don't have to move around on your desk, whether for comfort or for space-saving purposes, this is an excellent option. 

Trackball Mouse Comparison