I did it for a while in my truck. I figured- if I drive the way I really want to, I get about 16mpg commuting back and forth to work. If I shift early to save gas, I get 18mpg. What's the big deal about 2mpg? Then I did the math... 2mpg over how much I drive my truck in a projected year comes out to about $500-600 in saved gas. Half a thousand bucks can buy a lot of stuff, or a nice toy.
That easily pulled me back into line. It's not like the truck is as much fun to drive as some of my other cars.
Now the S2000- that's worth the 10% efficiency penalty to drive it how I want to drive it. I only go for max FE on long highway trips, and even then I still try to move at "normal Texas speeds" on the interstate. That engine is absolutely meant to live at 9000rpm anyway.