Earthquake-actuated gas shut off valve for California houses?

[Jim G.]

I am seeing a lot of CA folks on this site.  Would like to know your opinion on this type of gas valve.  I am living in Bay Area with a few rentals in the area.  Non of them has this type of valve installed.  I believe the damage from the earthquake would be bigger caused by aftermath fire than by the earthquake itself.  Given the high repairing cost, do you think it it is worth to upgrade all the house to this type of valves?     If you've done so, how much?  Any contractor that you could recommend?

-Jim