It uses a flat rectangular cartridge holding one disc per side, and the entire drive mechanism rotates around to each disc -- very cool. You can get at the front disc without completely pulling out the cartridge. It's possible to perform a complete four-disc swap in your lap while driving with one hand and without looking (once you've chosen your titles of course) with a little practice. (You could also keep spare cartridges and just swap out the whole cartridge, I suppose.) The cartridge itself is pretty rugged, with no swing-out arms or other parts to break off easily like many CD changer cartridges.
The entire front face is removeable. I once dropped the face (in its carrying case) from a second story balcony onto solid concrete, and while the LCD cracked and a couple of incidental indicators are gone, it continues to work fine. The unit is well-designed as a whole, less fragile-feeling than I'd anticipated.
It takes about four seconds to change from a track on one disc to a track on another. The whole time I've had it the thing's skipped once (on a potholed cobblestone alley in San Francisco), despite less-than-ideal road conditions. The 10-sec buffer works.
One thing about this deck that some people hate and some people like: several functions are not controllable from the head unit itself, but only from the remote commander (the 2.5-inch tall joystick-looking thing). Track change and forward/reverse scan and tuner scan (pretty essential) are two such functions. This is actually advantageous if you can mount the remote commander on or near the driver's door and away from the center console, because it means your passengers won't go messing with the stereo changing stations or tracks. (I recently got a new car and had to put the remote commander in the center console, and now my best friend thinks he has free license to change/skip tracks or entire discs whenever he wants. At one point I had to tell him to leave my stereo alone or get out of the car. He still messes with it on occasion, and I've kind of given up. But I suppose how this affects you depends on who your friends are.)
I know of a three-disc no-cartridge Sanyo deck that's been out for a year, but haven't actually played with one.
There's also the MDX-60 standalone six-disc change-while-playing no-cartridge MD changer, which is supposedly compact enough to fit in some gloveboxes or elsewhere in the passenger compartment. This, combined with a CD head unit, might be a good alternative, if you can get the MDX-60 mounted somewhere within reach. Of course, I don't think you'd be able to see the disc/track labels on the head unit that way.
The MDX-400 is really expensive, though; if you use MDs for music mixes you may not need the capacity -- an MDX-C150 single-disc head unit with 10-character LCD and CD changer with CustomFile support is about half the price and comes with a (weak) built-in amp for lo-rent installs.