I've been commuting 39 miles to my current job for almost three years. I've been taking a decent route but ignored the GPS route. My route sticks with larger roads, the GPS lops off a large corner by taking an empty country road.
I've stuck with my route because it takes me past some shopping I sometimes need, it seems more appropriate for commuting, and I was skeptical about the other option.
Today I went the GPS way. With no traffic, I didn't have anybody tailgating so I could go whatever speed I wanted and my stress was lower. The steep hills and low speed meant I actually got to use different gears instead of 5th all the time. The hills also helped my P&G...AND it's 2.5 miles shorter.
I'm not sure if it might take a couple more minutes, but it's a nicer ride (scenery and traffic) and should make a dent in my fuel usage. My FE was up a little today, and as I learn the route it should improve.