This is probably a bit behind the curve as I'm sure many of you have already got your hands on a copy of Battle Missions by now, but I thought I'd throw out my 2p.
I got to have a look at it a couple of weekends ago, and play one of the missions and I was very much impressed. Although most of the missions are specific to a single army, with two different opponents you have at least 6 missions to pick from based on the two races, a lot of the other missions are pretty general as well so could easily be adapted to other armies as well. I think its going to be a great addition to peoples gaming, far more than planetstrike was as it doesn't require any changes to your forces, the differences in the missions are all down to the special rules.
As to my mission, I was playing the surprise attack mission at 1000pts against a necron force with a monolith, lord, 2 warrior squads and 2 destroyer squads. I lost quite heavily as I just couldn't drag down the necrons. Stubborn thanks to the mission, and double We'll Be Back rolls thanks to the lord and monolith meant that I just couldn't wipe out anything to get any kill points, even all the power-weapons and rending couldn't help me with teleportation. Oh well, such is life.
All in all, I think Battle Missions is a good addition and I'll be getting one for sure.