Superior Mouse Control

I have been using the various iterations of the Logitech Trackman for many years. These trackballs are extremely precise due to their optical tracking system: The balls have small dots on them, and an optical sensor tracks the movement of these dots. There are no rollers to get dirty and there is no dead spot at all.
The ball sits on three small nubs inside the unit. Previous models had a small problem with gunk building up inside the unit and while the tracking was still very precise, the ball became harder to move. Cleaning the older units involved rotating a retaining ring (with a pen or other pointed object), then removing the ball and shaking or blowing the debris out. Logitech has fixed these problems on this new model with the addition a hole in the base of the unit allowing debris to fall out. Some clever shaping of the ball housing has replaced the retaining ring, allowing the ball to sit securely in the unit but making removal a one-step (pull) process.
The Logitech Trackman is extremely comfortable for right-handers, as the hand rests on the unit in a natural position. It is extremely comfortable to use for long periods, as no wrist movement is necessary. The buttons and scroll-wheel have excellent tactile feel, requiring little effort to use while avoiding accidental inputs.
If you only have one pointing device, the Logitech Trackman should be it. It is inexpensive and comfortable enough for general use, and precise enough even for photo editing and other exacting tasks. I highly recommend it.