Treating yourself to a better one than you currently own, is often as helpful to your daily routine, as adding more memory, a faster processor, ect. With the mouse handling the largest part of your interaction with your PC, it's a critical part of your overall computing experience. I've tried many mice over the years, and have settled primarily on Microsoft and Logitech mice, as my mice of choice. The quality of mouse construction is critical in my eyes to such a daily use device, and these two companies dominate this category, because they build quality. One of the reasons why quality is important, is that traditionally, mice wear out over time.
Most mice are what are called opto-mechanical mice.
You have a rubber coated ball, which drives rollers on x and y axes. These rollers in turn are connected to slotted discs, where optical sensors sense the open slots, and determine movement. All of these moving parts eventually wear out. This is greatly reduced, if you use one of the hard surface mousepads available now, such as the Ratpadz , but still periodically necessary. Last year, Microsoft released a line of mice that utilize CMOS imaging technology for movement detection, their Optical series.