For better or worse, the world of golf seems to have gotten it's measuring stick stuck 18.
But I grew up playing nine-hole courses. I learned to play on one, and I played in as many as 15 tournaments a summer, 90% of which were on nine-hole courses.
And now that play on an 18-hole course, I often play nine after work, or make a trip around one of the three-hole or six-hole loops it offers. (The truly masochistic can play 10-11-12-7-8-9 three times for ~7,200 yards.)
As Daryl said, some of my favorite times on the course have been knocking it around 3, 6 or 9 holes in the twilight--hitting a shot and then listening for it to land is something that cannot be matched.
K