My novel is going surprisingly well, considering I have no idea what I'm doing. My last two NaNoWriMo novels, and most of the successful writing I've done in the past, have all been planned to some degree, even if it's just a general sketch of the starting situation and a rough idea of what I want the climax to look like. This time, I mostly intentionally decided I wanted to do something different, and see if I could write my novel without any idea of what I was doing or where I was going. (I was also really busy this October and working on other things, but that's beside the point.)
I shouldn't be surprised, really, that it's working (at least so far, we're not out of week one yet so there's still time for it to all go to hell) because I do stuff like this all the time when I GM. One of the best single sessions I ever ran was almost completely improvised: a one-shot, Gnome Town. I'd sketched out a little dungeon in the hour before I ran it, but I ditched that half-way through and just started riffing on whatever bizarre thing popped into my head and whatever crazy plan the players came up with. The players were pretty hot that night, so it worked out well.
That's the only session I've ever run completely on the spot, though I did improvise most of my one session of Feng Shui. More often, I'll have a general idea of the situation, but little or no idea what the players are going to do. That tends to be my default play style, and it can range anywhere from reasonable responses to character actions based on the pre-defined scenario, to completely making up major details of the game on the spot just because it would be cool.
When it works, it works. Going into the Infamous Wedding Incident, I didn't have a clear idea of what was going to happen, figuring that the players would come up with something. They didn't have a clear plan, because they figured something weird would happen that they could riff off of. My solution? Faen ninja terrorists. In hot air balloons, with long-spears and flaming catapults. The players grabbed hold of it and started doing their crazy thing, and pretty soon we were jamming. It was great.
Of course, the session could have been even better if I'd had better floor plans for the wedding, and a better map of the city, and a better idea of the "normal" order of events, what would have happened if the wedding hadn't been disrupted by a four foot tall version of the IRA and a cross between Mr. Freeze and Jack Sparrow. And part of why it turned out as well as it did was that I knew who a fair amount about the other people who ended up involved--the town guard and the main villain--and what their tactics and motivations were. Because the faen just got the fight started; what made that fight great was the combination of Faen, the villain's attempts to take advantage of the disruption, one of the player's attempts to take advantage of the disruption, and the rest of the party's efforts to keep order along with the city guard.
But being willing to make stuff up as I go along? Being confident in my ability to make stuff up as I go along? It's been key to whatever success I've had as a GM. And it's fun. Part of the reason I count those sessions among my major successes is what I great time I had during them. Which isn't everything, because the fun has to be there for the whole table, but it's important.