One of my fav gadgets is trackball and Im unsure why trackball is not popular. It saves space, your wrist would say "thank you" if you regularly use computer for a long period of time, and it looks really
cool 
However, there are some downside. Its not good for playing game, you would swear the trackball to be got pwned
. The size of trackball is generally bigger than the normal mouse. It does collect dust, not as much as mouse though, but you do need to clean it occasionally