My biggest issue with Ion guns are that they are outranged by their natural enemy, the AT-ST, whose main gun is range 4. Having that sort of firepower bear down on your units whilst you attempt to get within range to fire your ion guns results in far too many casualties for the tactic of adding ion guns to 1-3 units. It's just not sustainable in my view. For 32 points per upgrade, you could take 3 Z-6s and mow down enemy troopers, who are usually the only unit capable of capturing objectives.
If Ions have been working for you, power to you. I just don't see them as a viable anti-vehicle weapon and certainly not worthy of their price tag.
Thoughts?