I'm probably in the 1% minority whom actually uses helicopters in my civ games. If my game does get into the modern age, I do build a few helicopters to allow me the flexibility to transport some troops quickly behind enemy lines, should certain situations calls for them.
But I can't say I've ever considered it to be the "best unit to build"...
Helicopters are just specialized units that expand my tactical options. At this point in the game, I don't consider the few gpts to maintain a small fleet of helicopters to be a problem.