At it's core, its a simple question: Do you run modules / adventures "as is" or do you tweak them to better fit your expectations and by extension, those of your gaming group.
I tried running a draft of Dwimmermount "as is", as that was the group's request, but we didn't have much luck with that.
We followed that up with the first few levels of Rappan Athuk, yet another megadungeon. With RA, I did tweak it a bit, but the tweaks were relatively minor.
Now we are trying our hands at the DCC RPG, and there are some extremely well written and original adventures from both Goodman Games and the assorted third party publishers - easily enough to keep a campaign going for a year or more. I expect to twist and tweak the threads linking the adventures and in turn tweaking and rewriting many of the adventures backstories, but I don't see a foresee a need to rewrite the adventures themselves, but time will tell.
How much tweaking do you do to the average "store bought" adventure or module?