Air attacks.

[Wdll]

In modern times they can easily win a battle for you. If done correctly, enough to make the opponent rethink if he can continue fighting the campaign against you.
What you need though are a few things.
a)Numbers. You need at least 10 planes.
b)Numbers. You need several missiles/bombs per plane. None of that 2 per plane crap, unless they are SEAD.
c)If the opponent has good AD, you need more than enough SEAD to go in first to either kill off or force the AD to waste their ammo on them instead of your strike aircraft.
d)Long term plan, at least in a PBEM campaign. You can never know when the game will give you the air sorties you will need, and the support points to go with. You need to save points from the repair points you get. Which means you need to sacrifice other things.
e)If the enemy AD is very good and easily outclass your SEAD planes, especially in EW, forget it.

Is it worth it to use planes? Hell yeah! But only if you are serious about them. If in a 45K (initial) army you get 2-4 planes and expect to win the battle with them...that's a waste of credits. You're far better off buying anything else.