In Rebal, I decided that it doubled the efficiency of PD against ordinance and fighters, rather than turning Tac Lasers into PD (which, honestly, is its only use case that's good).
It works great, worth buying for Captains you don't put Advanced Countermeasures 3 on, or when you need an anti-fighter / anti-missile boat in your fleet. Not really worth it if you've put Advanced Countermeasures on them, unless you really, really want to make the ship proof against incoming missiles (Dominator and Onslaught builds, for example).
I think it's pretty meh otherwise, honestly, but then again, I don't have Flares wasting valuable System slots, either.