i was thinking about using one instead of an encoder wheel once (i didnt bother with it eventually).
i thought that the optical mouse base needs to be in contact or very close contact with the floor which can cause some problems, i decided that if i ever do use one then i would remove the board and sensors from 2 mice and have the sensors close to and pointing at the wheels, if you calibrate your software correctly, they will work just like regular encoders only more technological.
also there are the 2 buttons to be salvaged for bump sensors and usually a scroll wheel