I have a Garmin 1490T and the saves gas and shortest distance settings at least in my Prius works very well at improving my gas mileage. I figure it's worth at least five miles per gallon.
I was going to Philadelphia and had it set up for shortest distance. Let me put it this way, my GPS took me through parts of that city where I kept looking around saying to myself I didn't bring enough firepower or ammunition.
Hint don't ever follow one of these units blindly when you don't know the areas your traveling though.