Do you use pre-published adventures or write your own?
I think that like most DMs, I use a combination. It's almost impossible to run a campaign and always use someone else's adventures all the time. Things happen at the table that make it hard to fit in pre-published adventures, making it almost necessary for a DM to do their own thing.
Having a module to work from doesn't mean the DM's job is done. It's a rare module that ever gets run as written. Even the most comprehensively written adventure isn't going to address everything that the players will come up with. No adventure survives contact with adventurers! Sometimes the adventure doesn't survive the prep before the game either.
For example, I've been running a slowly ongoing 4e game, running through the WotC published adventure path from Keep on the Shadowfell to the Thunderspire Labyrinth, to the Pyramid of Shadows. All three have been modified to one extent or another. Some of the changes have been minor, just tweaking monster stats or adjusting the layout of a room. Other changes have been fairly major, like redrawing the entire map. If my players and I weren't so interested in running the series, then we probably would have done something else, and I would write something a little more interesting, story-wise.
There are advantages to doing it entirely yourself. No one knows what you want, what your players will enjoy, better than you. It's much easier to run something you know inside out, because it came from your own imagination. Even if it's an adventure you wrote not in conjunction with a campaign, say a one shot for a convention or gamesday event, it'll be one that you are more likely to enjoy running.
The downside to doing it yourself is that it can be an intimidating undertaking. I know that when I work on my own adventures, I worry that my ideas aren't interesting enough to keep the players attention. I worry that I won't strike the right balance between challenge and reward.
So far I've found that the benefits of running my own adventures outweigh the downsides.
This is post 4 of the 30 Days of GMing Challenge.