I think it's a fascinating tool that probably deserves a place in diving history: a proper decompression timer that can be used as a timer underwater and a timepiece topside.
I've never used one, but would certainly be interested in trying one out underwater; it would be interesting to see how easy it is to use (I assume it's pretty straightforward, once you've reset the minute hand to twelve) but as I rarely go into deco (if at all) and hardly ever follow a classic stepped plan, I'm not sure I'd be able to test it properly...