The killstreak is unlocked after getting 7 kills in a row or 6 with "Hardline" attached. The attack helicopter seems to have been over-shadowed by the "Harrier Strike" for many reasons; the Attack Helicopter's main weakness is its lack of flares. A single direct RPG shot will take the Helicopter out the sky, as well 2 AT-4's can be used, or even the Predator Missile. It's advantages are that it circles the whole map instead of one specific spot and it apparently fires "FMJ" rounds, making its penetration through walls more effective.
Strategy
The Attack Helicopter is returned from Call of Duty 4 but this time it's been shoved to the corner by the more powerful killstreaks. This killstreaks is still extremely useful. On the larger and medium sized maps the Attack Helicopter will circle the map raining fire on the ground enemies. The Attack Helicopter is vulnerable to the Harrier's missile barrage, so this is why more players are going for the Harrier. But the Attack Helicopter is best suited on Free-for-All or Search and Destroy game where not my other players will have killstreaks in the air.
I seem to die by an Attack Helli every time it comes into play. Where I never used to, so maybe it is more powerful. Or I might be an attraction to airsupport...yeah...