I found and joined geekhack because of keyboards, but all the people using trackballs piqued my interest. So while I was picking stuff up at my local computer store during the christmas sales I grabbed a Logitech Trackman Marble for US$30 - decided it would give me something cheap to see what they were like.
I used it for a little while and found I really liked it for general computer use, but the DPI was just way too low for my 2x1680x1050 monitors (plus a dual screen computer controlled over synergy+). I also found the lack of a scrollwheel annoying So I decided it would be turned into a cheap mod project to get my soldering skills up to scratch.
I dug out my old Microsoft/Razer Habu which has dying switches and cannibalized it: Pulled it apart, took all the switches off and soldered wires to connect the 'new' switches and to move the scroll-wheel.
Then I pulled the Trackman apart, cut the switches off (connected to seperate boards with wires so this bit was pretty simple) and connected it up to the Habu's PCB. I also cut most of the plastic out of the bottom of the Trackman case as it was in the way.
At the moment I'm squeezing and glueing it all into place inside the Trackman case, I'll raise up the sides of the bottom part of the plastic case with some epoxy to make room for the larger pcb and the scroll-wheel positioned for right-thumb use (on the left side below the buttons).
The epoxy setting time makes it quite a slow process (especially since the stuff I've got needs UV to set so no night-time setting)... but I'll get there, and hopefully it's much nicer than it was before.
I've taken a couple of photo's and I'll take more and upload them all once it's finished.
