I've seen them fire but I'm not sure if they've actually hit anything. I put a plasma defence array on my probe since it can't carry much else to be useful offensively, but if it were to get shot down is there any point in making another and using it for missile defence again?

They are fairly good, but there's a very big catch. The main problem with them is that they don't pound out enough damage to get the alien missiles in time. I assume missiles have hitpoints of their own that the defenses attack. You need at least two, three or more for them to see any results, and maybe a missile evasion matrix to throw the missile locking off a little to give them a bit more time.

 

After all the preparation you'd need to set up an anti-missile vehicle, you might actually be better off with a shield and more firepower.

 

- NKF