Chopper Command is essentially a Defender clone, dropping the outer space setting for a desert military theme. The graphics and gameplay are streamlined to better suit the abilities (and limitations) of the 2600, and as a result, the game easily trumps Atari's home version of Defender.
There are no smart bombs and no worrying about your humanoids being captured and turned into enemies. You simply defend a convoy of trucks from enemy jets and helicopters, and once all the trucks are destroyed the game is over. The game "world" is several screens long, with a radar map displaying the big picture on the bottom of the screen.
The graphics and sounds are top notch, and once you've cleared multiple waves the game speeds up to a frantic pace. Yet for all its spit and polish, there isn't that much to the game, and I've never found Chopper Command to be all that compelling.
So I recommend Chopper Command only half heartedly. Many people seem to love it though, so your mileage may vary.