I've had the Duomatic, S2C and am now using the SRAM Automatix.

Love the Automatix. Shifting is great, braking works great (I've even removed my front brakes because I am totally confident of the braking). I was initially tentative about the automatic shifting, thinking that I preferred the control over when I shifted. However, I find that I can just slow down and get the lower gearing whenever I wanted.

The S2C sucks. Believe everything you read about its reliability issues.

The Sachs Duomatic was great, reliable. They're expensive and hard to find. Other than nostalgic value, I don't think they offer any real benefit over the SRAM Automatix. If you do want to open one up and rebuild it, there are many reference materials online to help you. I was able to do it, and I'm no mechanical genius. You just have to be careful and make sure parts go back in the way the came out

Oh, and wait for BruceMetras to chime in. He is the master authority on all these IGH's....