My 2 thrones:

The system is crud, without gaming the system your character is crud.

That said of the 3 big flag ship titles, Deathwatch is the worst of them.
Not because your space marines, which are FRIKKEN awesome,
but because your, well, a deathwatch kill team.
Go here, kill that. Go there, kill this. Go here, kill that. Go there, kill this. Go here, kill that. Go there, kill this. Go here, kill that. Go there, kill this. Go here, kill that. Go there, kill this. Go here, kill that. Go there, kill this. Go here, kill that. Go there, kill this. Go here, kill that. Go there, kill this. Go here, kill that. Go there, kill this. Go here, kill that. Go there, kill this. Go here, kill that. Go there, kill this. Go here, kill that. Go there, kill this. Go here, kill that. Go there, kill this. Go here, kill that. Go there, kill this. Go here, kill that. Go there, kill this. Go here, kill that. Go there, kill this. Go here, kill that. Go there, kill this. Go here, kill that. Go there, kill this. Go here, kill that. Go there, kill this. Go here, kill that. Go there, kill this. Go here, kill that. Go there, kill this. Go here, kill that. Go there, kill this. Go here, kill that. Go there, kill this. Go here, kill that. Go there, kill this. Go here, kill that. Go there, kill this. Go here, kill that. Go there, kill this. Go here, kill that. Go there, kill this...................................

This is all you do. Ever. If you do anything else, you are drifting away from what a Kill team does.

The best one? Rouge Trader. Why? Because you can do... ANYTHING. Everything is fair game, and no one can (legally) stop you. It is freedom personified.
The official song of Rouge Trader:
http://www.youtube.com/watch?v=k8F3UE9qFsg