Use Helicopters to transport 3 infantry units every turn from any of your cities to any other of your cities (or to simulate a 2 link Transport chain). I consider them superior to Transports in transferring defense troops to where they're needed. This is arithmetically provable in any instance in which you need more than 2 links in a ship-chain to send your troops to where they're needed. Of course, the Helis don't have to reconfigure, either for alternate landing areas, nor are they subject to attack when used to Rebase.
I've used Helicopters to take out enemy Rubber and Oil, reducing them to Guerilla and Cavalry I'm fielding in Infantry and Tanks. Basically, they're like Galleys and Transports, only better.
They're great for isolating enemy cities when dealing with bottlenecks (usually one or 2 tile bottlenecks). Likewise, they also add flexibility to troop placement tactically, if you have air superiority. There are times when you back off an attack because your troops might be placed in an awkward position after 2 consecutive attacks. You can use Helicopters to establish fortified positions even deep in enemy territory, a concern when you're using fast moving attackers like Tanks and Cavalry.
When you can build Helicopters (and have air superiority), you should try to avoid sending in SODs one tile at a time into enemy territory because the Infantry is so darned slow. Just send the troops in fast and pack the Infantry in Helis so they can catch up.
I tend to build at least 9 or 10 Helis for various purposes in a Standard map, though I've accunmulated as much as 20 in one game.