A trackball opens up a number of games that are pretty fun, Marble Madness, bowling games, other sports games, are just a few.
A good arcade trackball is $$$ though, and they take up a pretty good amount of CP space. But, they're USB and easy to hook up..
Also, make sure you provide plenty of clearance for an aggressive hand when using it or you'll be bashing fingers on the screen all the time.
But I'd say if you can swing it, you won't be sorry.
Now, the flightstick I put on my CP, that's a whole different story
