I attended D&D XP and played 2 of the 3 Ashes of Athas adventures (1 and 3). Unfortunately, I only had time enough to play those two and get home.
The adventures were well-done, not unlike the better LFR mods out there. Definitely had a storyline going through the adventures (I heard a little about #2 and was able to piece together what was going on). The players were on the experienced side and knew both Dark Sun and the 4E rules. I found the DMs to be good as well.
Ashes of Athas is an Organized Play campaign, but it is not run by WotC. Instead, Baldman Games (also the organizer of D&D XP) runs this campaign. Unlike LFR, Ashes of Athas does not have game-day, in-store, or home play options at this time. It can ONLY be played at conventions, and only those where Baldman Games will have a presence. I know that they will be at Origins and Gen Con and some others that escape me at the moment.