I haven't used that, but buying it depends on what you've unlocked via research in the gun bunker.
If you've unlocked it already there's a tow behind trailer that has missiles on it that can shoot down a jet. I've used it a couple of times and it'll shoot like 5 or 6 consecutively. Haven't seen a jet outrun them all yet. Only drawback to it is that it takes a long fuckin time to lock onto a jet. That time it takes to lock on can get you killed if the jet pilot sees you
But if you going after a jet the explosive sniper rounds are the best way to get it. All it takes is 2 direct hits, and the first hit actually will either leave it smoking or completely disable it, because it knocks pieces of the jet's flaps off.
The bike, like
@LSN said is the next. If you can get close enough to it you'll knock his ass completely out the air. Those missiles are fast and you can shoot a lot, especially since you can begin shooting them as soon as it locks on vs waiting for the lock on square to turn RED.