I usually go with the quad gun. Still getting 4 shots, and they're twin linked, and have skyfire/interceptor, and you can put someone with a better BS on it, and the rest of the squad can still shoot at the flyer if they have the range/damage output necessary. If you play Space Marines, I'm not sure it would be worth having the squad shoot at the flyer, though. Probably best to shoot at stuff you have a better chance of hurting and let whoever's on the gun do their job.
As Chaos, autocannon havocs make a good squad to camp behind an ADL with their champ on the quad gun. If there's nothing more important to shoot at, out of 8 shots hopefully 1 will be a 6.
Sorry, to answer the question: No. Flakk missiles never seemed worthwhile.