Just wanted to let you know:
In tonights session, we used the system. (With 4 in a row stable modification, as mentioned above)
What happend? The sorcerer was able to stabilize after being knocked down to -9 hit points by a couble of balistae bolts, making 4 saves in a row.
Our ranger managed not to die (stbilized himself), after being droped to -17 hit points by an advanced thoqqua. During the same fight, the sorcerer died, rolling a natural 1 for is second save, after being droped down to -11 hp.
All in my group agree that this system is brilliant. It adds so much tension and fun. Tension, because it is always dangerous to be dropped. Fun, because dying without even rolling a single die to prevent that, is not that funny. (Happend in our game four sessions ago. The main tank got hit for a total of 100hp during a single round. Going from full health to death, without even having a chance to become stable)
Summary: It works

Without this system, the sorcerer and the ranger would have been dead because of racing through the -10 zone. In this case it was just the sorcerer, but that was def. okay. Even better is the sudden haste to get people healed. (But that will change once sthe player of the Dragon Shaman is back, I guess.)