I abandoned the digital input ports in the VB code since it sounds like they will be going away in the next hardware release. The analog inputs can read a digital signal, so unless you run out of analog inputs, it shouldn't be a problem.
As I recall, all 4 digital inputs are bits in one byte. What I noticed is that the digital inputs don't fall back to 0 right away when you disconnect them. So if you're using a normally-open push-button switch, it will go high as soon as you push the button, but it takes a while for it to go low when you let go of the button. The card has to drain off the voltage before it resets back to low. If you were driving it with something that can provide either 5volts or ground, it should work quite well.