So, I played a couple of scenarios over the weekend, Galatos and Heraklion, both from the invasion of Crete in 1941.
The Combat Cards which were played included:
Rough ground - did stop 1 unit, but they were part of a 4 unit attack.
Motorcycle dispatch - an extra unit was deployed; useful but not game-changing.
Sniper - made a couple of kills.
Ambuscade - led to one attacking unit being pushed back two hexes.
Return to duty - this was a huge help in the Heraklion scenario, enabling me to bring 6 infantry units back up to strength (I played it with a General Advance).
So far so good - the cards all added a bit of interest, but Return to duty was especially effective because of the card it was played with.