I found this post about SRAM Automatix quite informative, http://forum.ctc.org.uk/viewtopic.php?f=5&t=62320
I have one in Motobecane Fantom Cross Uno, with stock chain ring and 700x25 tires i got 53 and 72 gear inches. It get me quickly across intersection and I can cruise easy at 20-23mph.
I installed second stock spring (from another hub) and now it shifts at ~15mph and 94 cadence. The only downside is that this hub can shift sooner to high gear when You hit some bump in a road.