I used to be an extreme "wing-it" GM. All the prep work I did on my game was setting-wise, not adventure-wise. That is to say creating nations, geography, deities, etc. Because I had the practice, I got really good at coming up with names, plot hooks, even riddles and prophecies off-the-cuff (to the point I had players who refused to believe it was spontaneous). Everyone had fun, yada, yada. Absolutely nothing wrong.
Fast forward to the release of 3E and I'd been doing Storyteller for a few years. I wanted to give the fully overhauled and "balanced" system a fair chance, so I ran the adventure path series. Man, did I ever tank on "Sunless Citadel", and somewhat on "Forge of Fury".
That, along with a couple of 1E/2E experiences made me very cognizant of the fact that DMing from a module vs. winging it are two, almost totally separate, skills. Some people are better at one than the other and either can be learned. To echo others, though, as long as everyone is having fun it doesn't matter what your style is.