I have tried both, and I have found the Optipac invaluable. One simple question: can you afford it? Then GET IT!!! Unquestionably the best interface for optical controls. The best thing is, you can hook up many devices to it. With the USB versions of the Ipac and Optipac, plus a USB hub, I'm able to interface everything in my cab (spinner, trackball, 5 joysticks, 19 buttons) using just one USB cable.
As I said, I tried hacking my own, using something like the "cheep spinner" design, and it never really worked to my satisfaction. Using a mouse PCB to interface a trackball is probably easier to do than making your own spinner, since you're only making a few electronic connections. But it might not even work after all your work is done. The Optipac most assuredly will, without spilling a drop of solder.
"The choice is yours. But choose wisely, for as the Optipac will save you time and give you everlasting joy, the mouse hack will take it from you."