I tend to start with prepub campaigns frequently...they save a LOT of time. However, the first session, I always ask the players what sort of game they're interested in. Which prepub I use is selected based on this, and I'll do some customization as I see fit to better match what sort of game they want to play.

I don't think I've ever run a prepub entirely without modification, but for some, Ive used a great deal of the encounters as is. I also have certain encounters I enjoy from specific modules that I crib from gleefully when the opportunity arises.