I bought my 05 Prius not because of any anticipated savings.
Yes, spending less on gas is fine, and the tax deduction is great too.
I bought the Prius because I believe we need to reduce our consumption of prime energy based on non-renewable resources.
I bought the Prius, because IMHO it is the first car that makes a significant step in the right direction, in a package that is extremely driveable, and user friendly, and reliable. It is a car that in many respects I would consder driving anyway, mileage aside.
In addition, I bought the Prius because the HSD is the first major redesign of automotive propulsive systems in a long, long time. It is the first ICE based propulsive system to address the issue of running ICEs at higher thermal efficiency. I want to support this approach.
I bought my Prius because hydrogen fuel cell proponents are barking up the wrong tree (unless HFC are combined with other technologies).
Reducing fossil fuel use is what matters most to me.
Along the same lines (sorry for the plug here) I get my home electricity from Green Mountain Energy 100% wind