I have only GM'd a maybe 10 times so maybe I am not qualified to answer... but, I enjoyed the prep work about as much as the game. I liked building the dungeons, finding the maps, making the puzzles, make the traps, drawing it out / sorting tiles, and programing for my digital sessions. It was all fun because I was doing it for my players and with them and their characters in mind. Its the same excitement I get when I am making my character to fill a role in a party. Maybe I am weird because the "work" was fun.
I haven't done it more because I was nervous wreak during the games, one group didn't enjoy it enough to keep playing (I blame myself, it was my first 4 games, I think I didn't give enough reward, it was a bit on rails because I needed some training wheels, and the party wasn't sure they wanted to play to begin with I kind of drug them into it) and the second group was enjoying it but one of them invited a GM friend to play as a player on my 10th game and apparently he is only happy when he controls everything so everyone quite to avoid playing at the same table and by everyone I include myself. I could not restart a new game because it became clear I could not invite that person or the person who invited him and I didn't want to start a new game without them because I would have to explain why they were not invited. They moved away, but I ended up as a player in another game because I am too nervous about how those two went to try with people I don't know well.
I was certainly surprised by my players a few times and designed my second campaign to allow a lot of player freedom, but our games were restricted to 2 hours for second campaign and just as I got setup to follow the party instead of hook and pull down a line, "the other GM" player showed up. Still kind of wish I could have used all my work. Maybe I will once I have 4 people I want to play with who are interested and willing to let me try.