The planet attacks...

[Ghost:Rosenkrantz]

Caused by eco-damage the planet launches horrible attacks right in the center of your faction.....
Base defensive units works bad now, doesn't they?

Later in the game an ultimate favourite unit could be:
Choppers with PSI-attack and defenses equipped with sopofiric gas pods . Due to their multiple attacks they can wipe out an entire native worm/launcher-attack-force, in just one run... (and maybe capture some)

[Clear Skies]

It doesn't work like that, unfortunately. Soporific gas pods only work against non-native units, i.e. they don't work against mindworms, spore launchers, IoDs, Locusts or Sealurks. Equipping units with Psi Attack/Defence also removes the possibility of equipping the unit with Hypnotic Trance/Empath Song - I'm guessing Firaxis saw this as giving the unit too much of an advantage.
Why use Psi Attack against native units? It seems slightly pointless, and doesn't actually make any difference to the assault as opposed to with hand weapons - as far as I know. Why not use hand weapons and equip with Empath Song, giving a +50% advantage?

In my experience, Psi Attack weaponry works best against non-native enemy units with poor-to-average morale. Watch out for Trained/Trance Sentinels - favourites of Deirdre. However, if your opponent has armouring technology that, when doubled, is greater than your weapon strength (that's making allowances for Perimeter Defences - for Tachyon Fields, triple it) then Psi Attack might be worth investing in. Couple it with Soporific Gas Pods - it makes a big difference.

[RedFred]

Great name Rozenkrantz, but the idea about capturing worms or spore launchers with any air unit is not accurate. Use a rover. If you don't capture, you still wipe out the entire stack with a single attack.

[cbn]

If running major ecodamage it is best to have an empath rover in enough of your bases to give comprehensive coverage. As Redfred points out, 1 attack kills the stack (and can gain a LOT of cash). I find the empath choppers to be handy in situations where you are not facing mass attacks of worms but instead
1. want to pick off occasional worms that might encroach
2. to pick off IODs.
3. to protect crawlers and explorers outside the heartland

So generally it is a chopper or two on the frontiers to protect the explorers but rovers do an overall better job againsta native life stack in your heartland. Always remember to have a couple of empath interceptors or empath Sam choppers so that locusts cannot commit a rampage before coming in range of the rovers.

[Clear Skies]

Empath choppers are actually useful, especially to pick off those IoDs which sit just offshore with spore launchers in them and bombard your improvements. Those really really annoy me!

[Earwicker]

Since planet attacks you from all angles, it helps to have a combination of "Wormer" units to knock out the worms. My favorite is a combination of 6r-1-move chopper and 6r-1-3 Tanks, both with empath song and high morale. The choppers take out lurks, locusts, and IoD's (oh, my!) and the tanks go after ground stacks all at once, as pointed out. A few strategically-placed artillery units with best weapon - anything chaos or higher doesmajor damage to worms - can soften up the stack for your troops.

If I have a ship with decent morale parked in a base, I'll use it either to bombard a stack or to try and capture sea units.

If you're running lots of ecodamage, it pays to have trance and armor on formers and crawlers to survive the initial onslaught.