I'd say 50% of DM fun is preparing the adventure before-hand (though this can be hard work). It's nice to go through your mind the evil ways in which you plan to torture the players throughout the course of adventure, hehe. Thus, if you have fun preparing for an adventure, the more preparation, the more fun. It's up to you.
I've had a few successful adventures where I've gone in-depth over every situation and room the players could possible stumble in, providing descriptions and references to monster statistics amongst other things.
Then, I've had a few good ones also where I just basically winged it, and made everything up as I went along, sticking to a basic storyline.
Yeah, preparation is fun. I'd also venture to say that the same goes for players, in the sense that creating a character or choosing new skills/spells, rolling hit die at level up is like 50% of player fun too.