I have hated those damn Kirby markers ever since I hit one on the fly with a perfect drive dead down the center that took a crazy 30* left bounce into a f*cking water hazard!
Seriously, I do like above the ground markers (little pencil thin ones on both sides of the fairway of varying color at 100, 150, 200, plus 250 on par 5s are good) because it makes it easy to do the math from the tee and see that I'm going to land around the 100 or 150 or whatever marker so I should pay attention to what awaits misses in that vicinity. On a course that's unfamiliar where the terrain isn't helpful to eyeballing it -- especially with a big drop shot teeball -- its really hard to tell know whether that bunker on the left is as easily carried as it looks or that I have no chance to reach the water on the right so I can freely swing with a driver.
Perhaps not such an issue for a private course where everyone will know it well, but its also useful when playing the ball from the next fairway over to allow one to eyeball the distance to perhaps +/- 10 yards and at least have a decent chance of putting it on the green. Once you start knowing all the distances from the wrong fairways on your home course without looking, its time to take up bowling!
Some courses use the little barber poles dead center at 150, and those are better than just sprinkler heads or just plates if you want to see where to go from the tee, and provide a nice aim line as a bonus, but someday I just know I'm going to hit one of those on the fly also, and I'm not going to be happy about it!