Just got back from running my first Deathwatch game. Things went well, the PCs managed to survive and get out with their gene-seeds intact. A good thing too when Hive Fleet Betrayer decided to show up on their Tau/Imperial swindig and start feasting.
The one big thing I took away from this? Hordes are freaking dangerous, and are going to make threat assessment for this game very difficult. The +1-2D10 damage on attacks can add up really quickly. Especially with weapons that are already doing 2D10+2 damage. It made the fights go weird as we had an almost party wipe, then what I thought would have been a harder fight just got wiped out in like 2 rounds. Still it went well and folks seemed to have fun.
For those of you looking to run Deathwatch though, just treat Hordes seriously, as they will mess your group up really well.