It's the culmination of everything Sega had learned with their 16-bit MegaDrive / Genesis, and though it's little more than a larger six-button MegaDrive pad with shoulder buttons it remains the gamer's favourite.
Sega didn't always know what they were doing: their very early pre-Master System controllers were basically rubbish, and Master System pads were little better: boxy and square and really quite dreadful. The first MegaDrive pad was a harsh affair that looked better than it played, but they started to see success with the MegaDrive 2 and its completely revised six-button pad.
It was probably made possible by Sega's status as world-beating videogame kings, with all the staff talent, manufacturing capabilities and - most importantly - money that entails. Sega put it all to good use with the six button MD pad and then, with the Saturn, nailed it hard to the wall and staked their claim as masters of the controller craft.
Well, for 2D games at least. It's very obviously not friendly to 3D games, as it's lacking any kind of analogue inputs (you can read about Sega's 3D controller solution if you like).
It also has the distinction of being the only six-button mainstream console pack-in controller. Microsoft and Nintendo sort of released their Xbox and N64 pads with six buttons, but neither can be considered a real six button pad. If you look at 'em, it's pretty obvious why. Of course if you lived in Japan NEC released the PCFX console with a six button pad, but since the system was primarily used for dating sims and sold only a hundred units or so, it hardly matters.
The Saturn pad is a simple affair. It's light but reassuringly solid to hold, but can be a bit rattly with six buttons moving around. As is normal for every game company not named Nintendo it eschews the cross-shaped d-pad in favour of a circular pad with an embossed cross on the top. It's not my favourite solution: while it's unerringly accurate for constant motion games like shooters and platformers, it is a touch too wobbly for tap-tap games. This is a fairly standard complaint for all non-cross d-pads though, and it must be said that the Saturn d-pad is probably the best of the alternatives.
The buttons are arranged in perfect order, though some users are offended by the smaller upper X-Y-Z row. During play the size difference isn't noticeable. Having six buttons on the face is a certain luxury when you're playing a game that makes use of them. Fighting game fans especially will appreciate the full featured layout. Lesser pads often forced the player to use shoulder buttons, or worse. Six face buttons and a pair of shoulders is a far better solution than four of each*.
There are several versions for the different Sega Saturn consoles (Sega, JVC, Hitachi, Samsung etc), though they differ only in their colour schemes.
It's a really good pad. It is, in fact, a great pad, one of the greatest. It is not, however, my favourite. There are at least two other pads I'd use over this one, and it's all because of the d-pad. If this had a Nintendo cross-shaped d-pad, it'd be the best. As it stands, it's top five.
If you play fighting games and have a Saturn or Playstation or PS3 or PC or Mac or etc, get one.
Sega saw fit to release this very successful design years later for other platforms. PCs and Macs got a USB version, and the Playstation 2 received a dark purple special edition for the release of Capcom's Darkstalkers collection. These new versions were identical except for the shoulders, which used smaller switches and a piece of foam rubber to create a much more satisfying clicky response. By way of validating the regard this pad is held in, these re-released pads sell for very high prices, when they can be found at all.
The Saturn pad is quite simple inside. One chip, a custom Sega part that is nothing more than dual 74153 ICs, and nine wires (Schematic here). The face buttons are standard plastic with rubber membranes over a PCB, with a rubber Start button. Like every controller ever made, basically. The shoulder buttons use vertically mounted buttons, the same sort you'd find in a computer mouse.
Saturn d-pad, cross section.
As the user tilts the pad, a cross-shaped piece inside pushes against the membrane.
The d-pads are interesting, though not unique to Sega. The pad is mounted on a short stalk, which is attached to a cross-shaped d-pad internally. This means the top of the pad, when removed from the base, has the d-pad attached. Most other circular d-pads use a single piece design, where the d-pad inside the shell is large than the opening it projects through, so cannot fall out. If we can assume for a moment that Sega didn't want a Patented Nintedo cross-shapped d-pad then this mechanism is surely the best way to realize a floating pad design. Microsoft's Xbox 360 pad is similar, but uses screws to connect the two sides, and SNK's Neo CD pad just has a pad and stalk.
There are many alternatives to this popular pad. The Saturn controllers themselves are plentiful, though becoming more scarce. I have heard that there are Chinese clones on ebay from time to time. Some hobbyists have created and sold USB-based replacement PCBs so that original pads can be converted to modern uses. And there are the re-releases, of course.
There is also a Japan-only pad, released by ASCII to support the Playstation 2 version of Seaman, which is remarkably similar. Called the SeaMic Controller, it has a removable microphone, six face buttons and two shoulders, a d-pad that is either licensed from Sega or a shockingly blatant clone, and - surprisingly - an analogue stick. It's a very Saturn-like modern alternative, and certainly a fabulous pad in its own right.
ASCII also released several very similar pads for the Dreamcast and Playstation, though they were much thicker and the DC version contained an internal motor for vibration feedback.
There's not much to go wrong in a Saturn pad. Watch for kinked wires that may indicate shorts or severed internal wires. Check the connector, which can sometimes come apart. Watch the buttons and d-pad, they should have a definite feedback, with a soft clicky feeling and springing back when released. If they feel flat or stiff, it's likely they're very worn out.
* Thanks austere.