well, when i first approached it, usb wasnt really an option. i really liked the DIY aspect of the parallel port adapters, (up to 8 pads). plus, you get 14 inputs per digital pad and none of the funkyness of a gravis gamepad hack. you also open up possibilities for console connections instead of emulation.
unfortunately, on the computer i wanted to use, the interface sucks up too many cpu cycles for me to enjoy my games. but breaking it down costwise, it was about $20 for 4 digital psx pads and parts to make the 4-way adapter. for 56 individual, simultaneous inputs that aint bad at all..