The Kawai K5000 is often cited as a bit of an undiscovered treasure. Using Additive Synthesis as the basic for many of its sounds, makes it an unusual synth â€“ most around at that time were either Subtractive or Frequency Modulation (FM), such as Yamaha's DX range.
Additive Synthesis allows you to create waveforms from the blending of multiple harmonics to create unusual sounds. It never really made it to the mainstream of synthesis due to it's rather mathematical mode of operation. However, owners of the K5000R or K5000S swear by it for use in ambient and ethereal music.
RetroSound72 has posted this 9 minute sound demo on youtube to give an idea of what this machine is really capable of.