Wizards obviously has all the sales numbers but we have to remember that any new adventure is sharing shelf space and sales with other books. A better look at the health of D&D overall is total adventure sales in any given month.
That being said, I think that too many of the adventures are basically the same. I hadn't bought any of the hard backs until DotMM as I liked the fact it was separate dungeon levels with minimal "save the universe" type story line. I can strip in for levels/rooms/ideas as I see fit.
That and it was on a big sale on Amazon. Total price after discount was about $18. I can give a 300+ page adventure a lot of slack for $18.
Wizards will have to start with some truly interesting concepts to get people enticed to buy the latest adventure rather than something already out there. Soon, I can imagine seeing used copies of adventures for sale in the FLGS.