The release dates for the map packs have (supposedly) been leaked on the German Xbox dashboard:
Crimson Map Pack - December 10th
Majestic Map Pack - February 25th
Castle Map Pack - April 1st
I tried matchmaking in War Games, but it hasn't quite grabbed me yet. I've bought all of the MP map packs for Halo 3 and Reach, but I'm not sure yet if I will here.
I finished the campaign on Heroic, and now working through it on Legendary. I liked it. I love the Mantis.
I really like Spartan Ops, though. It's like an extra campaign (which is good for me, since I really like solo stuff). Some have complained how there's no more Firefight, but I think something structured like this is better than bad guys just spawning on a map (and I loved firefight). I finished the first chapter on Heroic, then again on Legendary. I'm about to start on Chapter 2.