I had an 18 mile commute. The first 10 miles were along a lake, with rolling to steep terrain. The last 8 were in "town" along a busy 4 lane road / highway. I disliked the noise and the scenery upon entering town, but felt safer as the road had essentially an extra lane as the shoulder - and I had it all to myself. The tight, rolling country roads were fun to ride on the way home - but I never quite got comfortable in the pre-dawn darkness.
I think 18 miles was a bit much. I had a 1 1/2 mile climb to get to the house - fun on the way out in the morning - but demoralizing when getting ready to leave the desk at the end of a long day.
Perfect commute - 10 - 15 miles. Rolling terrain. Small towns.
My commute now (when I cycle it) is just over a mile. Not really a commute to get excited about...