The primary virtue that has contributed to the success of the current generation of optical mice is their ability to be used on a wide range of surfaces.
The first mouse I ever used, in the late 1980s and early 90s, was a three-button optical mouse that required a dedicated mouse pad. The pad was of a highly reflective metal, with blue lines running in one direction and grey lines perpendicular to them; the mouse tracked its movement on the pad by noting the lines as they went by. (It was thus possible, by rotating the pad ninety degrees, to achieve a mild analogue of the surreal experience described in Stratton's and Kohler's experiments on inverted vision: side-to-side movement of the mouse would then produce vertical motion of the cursor on the screen, while back-and-forth mousing moved the cursor from side to side; cf. the Jargon File entry on drunk mouse syndrome.)
In the mid-1990s at the University of Toronto, I sometimes used a computer lab that had Sun workstations with similar optical mice, an experience that drove home to me one of the principal disadvantages of the design: because these communal mouse pads received much heavier use than my personal one, the lines had mostly worn off, leaving only a small area on the periphery of the pad where one could mouse reliably.
<GTKY>
My own ability to mouse in that lab was saved by a design flaw of my own first mouse. It plugged in to the left side of the back of my computer, and had a rather short cord; as a result, I became a left-handed mouser (even though I'm right-handed for most other tasks). In the university computer lab, the mouse pads were bolted to the table, and there was exactly one computer with the pad on the left side; this was the one that received the least use, so the lines on the lefty pad were in relatively good condition.
wertperch writes: I used one of they mice - the pad was gridded metal. Was on a Unix worskstation. Was horrid. Computer Science lab. UCK.
</GTKY>
The optical mouse developed at Xerox PARC was not dependent on such a specific mousepad; it would work more or less effectively on any surface with nicely contrasting visual information on it, such as a page from a telephone book.
Present-day optical mice have become the thing to have thanks to incremental improvements on this early design; they are now effective even on nearly featureless surfaces. They are also, of course, a novel convenience to those who started out with the sort of mouse one has to turn upside down and clean the lint out of.