Flak is what you should buy if you have lost air supremacy. Flak is also what you should buy if your still fighting for air superiority. In the first case you will need flak to protect your panzers and artillery. In this case flak will be your only hope to have some chance of successfully going on the offensive. In the second case you will be smart to add some flak guns to your airfields since it will help you win the battle for air supremacy.
Flak in range of a hex under air attack is always used in defense. Flak I and Flak II have a range of 1 hex (so they provide cover for their neighbor hexes as well), but Flak III has a range of 2 and Flak IV even a range of 3. When you have lost air superiority it is much advised to research all the way to flak III. The extended range will really make a huge difference as overlapping fields of fire will emerge.
Flak stationed outside the hex under attack cannot be destroyed, but only fights at 50% of its regular strength as a trade off. Flak stationed in the hex under attack can be destroyed and often will be.
When your enemy is employing strategic bombers you might want to put some flak in your vital locations to protect them from the worst effects of bombing.
Flak is not that lethal to enemy aircraft and mostly causes readiness damage and retreat-hits. However it is when flak defense is combined with fighter intercepts that flak really shines. By diminishing the readiness of enemy aircraft they make them easy targets for the defending fighters.
A side-advantage of flak is that is has also some value in an AT-Gun role, they are not an 88 Flak Gun, but they do have a little punch.
This also concludes the first season of ATG college. I will bundle all lessons in a small booklet in the comming month. I think this booklet will be a very welcome companion to the existing manual, especially for players new to ATG!
Thank you all so very much for your attention! And for hanging in there throughout all these lessons!