I kind of like the idea of having stealth assassins sneaking around an slaying enemies while unseen; which is what ghost armour can do. But I haven't had much success so far.
The problem as I see it is roughly this: a soldier needs to be pretty well trained to use the armour effectively. (The need high psi skills, and high melee for the dagger.) So ghost soldiers are not simply throw-away cannon fodder. They need to be looked after. However, the protection of ghost armour is very low and the invisibility effect is unpredictable (as far as I know). So... maybe the ghost soldiers don't get shot at 7/8ths of the time, because of invisibility, but in that other 1/8th, they just die.
Ghost soldiers die easily to a stray grenade, or reaction fire. The invisibility effect is strong, but without some way of predicting whether or not the enemy will be able to see me, I can't see how to use the armour without very high risk of getting the soldier killed.
I'm interested to hear how people are using their ghost armour, and whether they think it's useful.