I mostly write my own adventures, but some of the time I run published stuff... and some of that stuff is
long (Return to the Tomb of Horrors, I'm looking at you!).
I do pull a lot of stuff from 'what has gone before' in my campaign, and some of that relates to later plot threads.
Let's see- since 3e came out, I've run the following published stuff:
Return to the Temple of Elemental Evil (about 20% of it- the group just dabbled in it)
Of Sound Mind
Return to the Tomb of Horrors (skipped the first about 15%)
The Gates of Firestorm Peak
Now, figure that we've prolly averaged 2 games/week since 3e came out, and you get the idea that we've gamed an awful lot.

So, given the quanitity of dnd in my group, that's not a lot of published work. (Most of it was significantly altered, too, with the exception of OSM and GoFSP.)
Edit: Oh, yeah- why do I prefer to write most of my own stuff?
To me much of the joy of dming is campaign construction. I love building monsters, I love weaving convoluted plot lines together, I love tying it all to the pcs. Modules both leave less room for this and also don't usually fit the setting quite as well either (in my experience and opinion).