I'm quite skeptical on any cost savings, since I still need a car. Also, since I don't drive it often and it's there, my kids take it to school. School is fewer miles, so I might save 10 miles or 1/2 gallon or $1.20/day (maybe $120/yr.)
I think the good thing is that I'm pushed off or reduced the need for car #3, and that's saving me real money, given that I have a 17, 16, and 14 yr old (who needs to be driven countless places).
I don't know if we'd have already gotten one if I always drove, but I do know that we're not even looking or considering it now. Friends in similar situations have that third car.