When I saw this on the official WoW website, I sort of rolled my eyes because the Armory hasn't updated my Shadow Priest's gear or spec in months. How can it possibly keep up with the calendar? I did a little poking around with it, and I was pleasantly surprised. I created a new event in-game on our guild calendar, and it showed up on the Armory about ten minutes later. I accepted the event on my alt on the Armory, and that showed up in-game almost instantly. If the features keeps that kind of stability long term, I will definitely appreciate having it. Little conveniences like this really help the community feel of guilds.
Now to bug people about integrating this into Facebook and the like.