George,
I personally feel that undulating greens offer more choice and options around the green. They demand that a player possess many skills in order to save par repeatedly. On subtler, more flat surfaces, players are more likely to routinely use the same bump-and-run or pitch approach, whichever suites their game best.
Conversely, severly rolling greens seem to add a detrimental element of luck to approach shots. Well struck shots can miss their target by a foot, and catch a downslope that sends their stroke bounding over the green. The same holds true for slopes that kill a bold approach landing just short of its target. I have no problem with penalties resulting from less than stellar play, but here, I don't think the punishment fits the crime. However, on recovery shots, these very same slopes are very useful in trying to kill a pitch that would otherwise be unstoppable. Same can be said for downslopes when faced with plugged bunker shots.
Ultimately, at the end of the day, the fun factor of well contoured greens wins the day, because luck is part of the charm of the game, and in the above case, does not affect play at the same frequency as do interesting surfaces.