The Sigma was one of the first synths i bought, and when i knew next to nothing. It must be one of the wierdest designs there is and at the time I probably thought, "ah cool! so this is what a synth is!"
I must confess it doesn't get a great deal of use these days and I even thought of selling it, but found a beautiful gltch, which seemed musically quite useful.
The thing I like best about it is that its performance orientated. You can mess with the two joy sticks at once and the assignable aftertouch is good. Its a good synth to play drunk as its so wild. But I can't really rave about the sound. I much prefer the Delta soundwise.
"On the following day , the sorcery undespairingly continued: I changed my series, chose other sequences, cut other lengths, spliced different progressions, and hoped afresh for a miracle in sound." (Stockhausen)