We use Combat Awareness a LOT. Essentially, if something takes place in the battle that is not in a PCs immediate field of view, we use Situational Awareness-Combat to determine how appraised of it they are. We use it so much, that even our Magician-type classes have it to quite a high level.
For PCs that don't have the skill, we use Alertness instead.
Typical situations that might require a roll are:
Attacker moving in from the flank or rear
Archers taking up position or firing from flank or rear
One of your colleagues defending your flank or rear coming under high pressure, needing assistance, or suffering a wound
Spotting the caster of a spell that has just targeted you or a colleague
and so on
In the attacker moving to flank/rear, a successful check negates the +20 Surprise bonus, and if the PC has won initiative also would allow them to declare a small amount of movement in the snap to perhaps negate the rear/flank OB modifiers also.
Similarly, spotting an archer allows Parry Missile to be declared, and allows quickness DB to be used, and so on.
A non-aware (surprised) PC is a sitting duck - my players have learned to develop this skill fast.
The Awareness checks don't use any activity of course.