I like the K1. There are a few things that it does really well, aside from taking up a unit in the rack. My favourite type of sound is the very (very) low notes using patches based around vocal or string samples.
The K1 was built in three versions- keyboard, rack and module. The rack version lacks the joystick editing control. The original operating system contained a bug which prevents an external editor from changing the number of sources (or is it muting sources?). This was put right in a later operating system upgrade (v. 1.1 I think)
One feature of the K1 which is very useful but seldom found elsewhere is the keyboard scaling to frequency control. Normally if you detune two sound sources the higher the note you play the more extreme the detuning. The unpleasant result is that the higher the note you play the greater (in Hz) the difference in tuning is; what sounds like a chorus of flanging effect on a low note turns into a fast vibrato on a high note. The K1's keyboard scaling to frequency control helps avoid this. If you set this control to slightly more or less than 0 on the detuned oscillator (depending on whether the detuning is up or down) you can avoid this effect.
I have placed 3 zip files of K1 related resources at this site. They are: