[narwan]

Don't fire them during the opponents face (set range low) unless you expect a valuable target to appear that turn that you realy want to take out. The disadvantage of using them in OP fire is that ATGM firing flashes tend to get spotted easily. The enemy forces then have the rest of their turn to take shots at your vehicle if it's spotted. You can't move it out till your own turn so until then it's a sitting duck. If you fire it in your own turn you can then immediately withdraw out of sight so the enemy can take far fewer shots at you.
At experience 75 the ROF of an ATGM goes to two. So if you have good quality troops they can get off 2 rouns before having to withdraw. Then they can certainly be worth their money as the missiles can often kill just about anything on the field.
When setting up an SP ATGM put it in dense terrain. At least wooded, rough+wooded if you can. It'll make them much harder to spot and hit.

Narwan