Improvisation is obviously one of the main skills that a GM must have. The problem is that no matter what the GM plans out, the players will do thier own thing - and more often than not, something completely out of the blue. I've had to improvise entire sessions because of some wild (but believable) tangent that my players have gone on. I presume that this is a moot point for most, as it seems most of you have already agreed with this view.
For my own part, during my Star Wars and Call Of Cthulhu sessions lately, I've been taking to improvising elaborate consequences to rolling a natural one on skill checks. A few examples come to mind:
1. In COC, my players were trying to escape a sealed cemetery filled with the walking dead. One of my investigators wanted to throw a rope over the wall to another player on the other side and climb out. Well he rolls a one, so I decide that it means he threw too hard and the whole rope goes over the wall. It be came a running gag, because as it turned out, he rolled ones on two subsequent Rope Use rolls in the coming weeks.
2. In star wars, my rope using player from above plays a low level jedi who has begun to try to train himself during the rebellion era. He's on some back water world and wants one of the small training bots, like the one Luke uses on the Millenium Falcon (the litte round orbs that shoot at you). Well he rolls a one on a gather info check, and essentially says to a shop keep "Do you know where I can purchase an orb droid for Jedi Training?" As any SW GM knows, it's very dangerous to be a Jedi during this era. Sufficed to say, some undesireable people heard him, and what ensued was 2 - 3 hours of a completely improvised sub plot involving keeping those who had heard quiet.
3. Last night, the SW campaign again, same guy (boy is he unlucky!), botches a search check in the ruins of a giant ship that crashed on Dathomir long ago. He's poking through the dark and sees a small, round, oval shaped object. He ponders it for a moment when it cracks and out pops the head of a baby Rancor. Unfortunately for him, he didn't see Mommy and Daddy until it was too late and he ended up being propelled across the room.
There are more examples, but these are the best. Frankly, I feel that the best moments in my sessions are the improvised moments, and would not only want the GM to improvise, but i would expect it. If a GM plans too much, there's a possibility that he can force the characters into a situation over which they have little control - which isn't much fun at all.