I have used helos with astounding success in a recent game with a war against the romans, then later the french:
Romans had northern plains and desert, suitable for armor exploitation. But their southern region was a moutain range. In order to attack it, I used helos to ferry marines and infantry to the outskirts of the city, then attack. I never saw the "Air drop failed" thing (but that was version 1.17, don't know about 1.21, if so then they have reverted to near worthlessness). The only editor mod I made that is realivent here is I gave infantry an attack capability (A & D are roughly equal). If the romans had rubber

, I would have been in for a tough fight, but against just riflemen it was a cake walk.
Later, I regrouped my helos and loaded an infantry unit on to each. When then when I launched my operation against the French (destroy a major power in 1 turn) I used these airmobile infantry and marines to take 2 major cities.
They have their uses, but I agree with a few complaints:
units should have a MP once hitting the ground, at least if they land on plains, desert, grassland, etc. This should also be true of of airborne units.
Helos should not have an issue landing troops in rough terrain, as proven in Vietnam, their will be clearings enough to land several troops in even thick jungle, hills or moutains. Heck - just use a daisy cutter in a jungle, instant LZ
Helos need to be able to recover infantry. Right now, they are a one way ticket. They need to be two way.
Helos need to be able to base in different territory:
helos with troops should be able to base on carriers (or assault ships), or ground, at least just include friendly territory, but they should also be able to set up operations even in enemy territory.