So, the vasari Overseer, which everyone seems to think is useless...
This is the only unit in the entire game that has a direct hull point heal. It has a cooldown of 2 seconds and repairs 250 hull. That's 125 dps mitigated per second with just one overseer. Keep in mind this is DpS that is hitting the most heavily armored race in the game. This is healing the hull that is getting depleted after the incoming fire has overcome 50% shield mitigation and the additional 30% or whatever that your armor is providing. Also, the ability buffs the armor of it's target by 2, which is something like 8% additional mitigation. Ever wonder why the vasari repair platforms repair way less then any other repair platform? Because vasari armor is F-ing sick. Vasari hull points are just -worth more- than other races. (This is also true because Vasari are cooler then the other races, but...)
They have enough antimatter to chain-cast this ability about 5 times before they're completely out of anti-matter and more or less useless. This means that is something is taking 125 damage per second straight to hull (so basically if their entire fleet is focus firing whatever you're repairing) then the overseer will be spent after 10 seconds of casting. They do have tendency to blow their wad pretty fast. But, their 'wad' while chain casting is 1250 hull points. If they're repairing an enforcer, or a cap ship, or the subverter that just rendered half your opponents fleet useless, then they are gold. The auto-cast functionality is pretty good, but it is highly dependent on whatever in your fleet is getting shot at. If your opponent has enough firepower to drop an assailant in one volley, you might see a lot of wasted healing. But pretty much anything under that mark an you're good to go. I cannot stress how amazing these things are at keeping up capital ships that are getting focus-fired. Three overseers and you've doubled the life expectancy of your capital ship, or effectively turned a smaller ship into a capital ship simply by turning it into the ship that will not die.
I definitely don't recommend having a lot of these things. But carrying around 3-4 is -DEFINITELY- worth it. The hull repair is nothing short of amazing, and extra recon on enemy forces is pretty sweet. Knowing when a force you can't possibly take is about to jump into orbit around the planet you're nuking gives you time to turn tail and pick a better fight, and their slow jump-in thing can give you extra time to flee... Although that is definitely the least useful of their abilities.
Anyway, I have found that keeping a couple of these ships around is pretty awesome. They definitely make up their cost in supply. But more than 4-5, no matter how big your fleet is, and you're probably just wasting space.