Over the years, I've had a number of electronic PDA's, going all the way back to the classic Palm m100. Before that, I'd gone through several of the small binder style organizers like you find at office supply stores. None of these really worked well for me.
The binders were all too bulky to be truly portable - I wanted them for day to day use, not as a business tool.
The electronic PDA's were considerably easier to transport (except for the one with a built-in keyboard), but it was a bit of a pain in the butt to enter anything. The Palm Graffiti system would frequently misread my stylus strokes, often leaving me wondering what the hell it though I had just done.
Portable, flexible, cheap...just about perfect. The only problem is that the cards tend to get pretty bent up in your pocket.
Suddenly, inspiration struck (I've noticed that it never strikes gradually).
I cut up the cardboard backing from a tablet, added a bunch of Duck Tape and voila!
Portable, flexible cheap, and now durable. A few more geek points for me.