As is normal for controllers seeking to avoid Nintendo's d-pad patents the PC Engine pad used a round d-pad with a cross-shaped relief. Unlike other attempts to replicate this mechanism (like Sega's execrable Master System pad) the PC Engine d-pad was comfortable, accurate and reliable. It is no less player-friendly than the Nintendo d-pad.
If the ergonomics of the entire pad are taken into consideration it's very likely that this is, in fact, the very best 2-button controller ever made.
It was unique in at least one respect: it's the only major manufacturer's pack-in pad to include auto-fire. Every single PC Engine pad, for every revision of the hardware (barring a rare standalone version of the PCE pad, shown below top-left), had 3-position autofire for both buttons.
The design is so solid that, for most of the PC Engine's lifespan (which was very long indeed, with game releases spanning more than a decade) only the very last hardware revision - the Duo RX - had a different pad packed with the system. In fact, the PC Engine pad ranks at the top of the list for successive hardware revisions that used the same pad: more than ten revisions of the system were released worldwide, and each used the same controller. To keep it from being entirely stale, each featured a different coloured plastic shell, a different label panel, or both.
Of all the PCE hardware released, only the Duo RX and the PC Shuttle had pads that were different in shape or function. The RX had a six-button arcade pad, and the Shuttle had a strange bat-winged pad that was functionally identical to the regular PCE pads.
NEC did release three other pads for the PCE series: the Avenue-3 pad, with a third button (which was simply a duplicate of the Select or Run button) released along with Capcom's Forgotten Worlds, the Avenue-6 pad, which was an uncomfortably large six-button pad, and the Arcade-6, which was such a beautiful pad it was later re-used for the PC-FX console.
The PCE pad uses the same chip (a 74157)as the MegaDrive / Genesis pad, though the two controllers are not compatible. A second chip (a 74163) is used for autofire. The schematic for the controller is very simple, as you can see.
Interestingly NEC used some very unusual resistors for their PCE pads. Rather than the usual components, these appear to be sprayed onto the PCB (below).