My own take on shield units.
It doesn't seem like firing out of them did damage, but I might have missed that(if it does, at least their own attacks shouldn't cause damage to it).
Edit: Ah this appears to be a feature of the new version, I still contest that their own shot should not hurt themselves.
Damage wise I think they are useful, they are decent anti fighter units, meaning they can be paired with cruisers as good sized force multiplier--they both protect from and kill fighters. As long as you keep your cruisers out of range of most other targets(and take out enemy cruisers with your own fighters somewhat fast), they don't get overloaded fast--and the cruisers kill bombers before they get into range, their main weakness. A few more hps wouldn't hurt, but it seems that almost all specialized units have lower hps than the normal units by choice.
2nd, logic wise it would be great if they automatically spread out to minimize overlap. I was running a group of them and cruisers yesterday and they tended to all clump together. While the army was smallish, I was manually moving them around to protect the cruisers rather than all clump in the center protecting each other while the cruisers hung out by themselves.
Not sure how to achieve this, perhaps some sort of minor negative weight for being inside the shield of another shield unit when their are units within x range not covered.