I still use it. In fact one of the reasons I chose my D90 is because it has spot metering. Another is the L-O-W price for this fantastic camera, yet another is ... NM - Lets just say I LOVE this camera
. Anyway - I find the zone system not only relevant to digital, and not just because the very excellent matrix metering system Nikon uses is based upon the zone system, but because the concept of detailed blacks and/or not blown out lights applies to ANY photograph taken by ANY camera, digital or film. One can pooh pooh the system, but lets not forget it's purpose is a pleasing image - and that should appeal to anyone interested in IQ. After all - a smart camera is still just a tool - the camera and computer between your ears is the one that matters, and if you don't know what your camera doesn't - however do you hope to master it rather than be it's slave? Did I say I love this D90? Still further off topic (sorry) I have no idea why people bother with Canon after using this - Nikon is in a whole 'nother world.
Also - Nik software's Silver Efex Pro 2 has a small section devoted to the Zone system. Simply mouseover one of the gradients and it is highlighted in the image to better help you determine where you want to be overall.
**Edit - yes, The Camera, The Print, and The Negative are still in print - you can find them on Amazon