One week later review :
I've finally tested the ass-saver in light rain and in wet ground yet not rainy conditions.
It works as advertised, i.e. kept my back dry.
My only complaint is that fitting it to a San Marco Light Condor saddle can be quite tricky, the ass-saver bends sideways and I can't fit it under the saddle when folded but that's no bummer as it's back in my bag in less that 2s (beware, already got one stolen because I was lazy and left it hanging beneath my saddle).
Good job guys, nice concept : simple, efficient and not too ugly :) !