I'll try to put it as shortly as I can
mearls said:
If you use published adventures, why? Are there any titles in particular that you enjoyed running?
If you don't, why do you find yourself creating your own adventures as opposed to using pre-made ones?
In general I prefer to create my own adventures. Main reasons are:
1) if I have written it, I know it much better and have no problems in remembering anything
2) I can put the creatures, locations, situations, events etc. that I like most, without having to use stuff which doesn't fit with the current campaign (which can be changed of course, so it depends how much is unfitting)
3) there is no chance that any player has played it before, or knows what is next
Unfortunately, it takes time and effort to write down a good adventure, so I often use published ones as well (mostly borrowed rather that purchased). In that case I cope with what I don't like and normally use it as it is (which means that the players who know my preferences well can be surprised this time to see something I never use), or change what really I dislike. The advantages in this case are
1) smaller effort to prepare the adventure (but more effort to run it)
2) advantage of having maps or similar aids ready
3) there's a small chance the adventure was playtested and is therefore balanced, otherwise I'll probably spot some reviews about it
More or less, it's about half custom adventures and half published ones, depending on how much time I have. There are also some non-commercial adventures written by RPGers which are extremely good and free

(take a look around for Zappo's Abyssal Campaign)