These are the evolution of Zerg recons play games . Between the probe's passive scouting, and raven's aggressive functionality, the overseer is swift at gathering information and brings the magnitude of your swarm to a higher level.
You can quickly evolve your overlord to the overseer, after researching a lair. They spawn into flyers comparably well-balanced as opposed to the recon units of other races: Overseers have high hit points and the armor support needed to pass and retreat from enemy borders, unlike probes, which often immediately meet its demise once detected. Comparing to the ravens of the Terran fleet, they are relatively inexpensive to train-about half the resources required for one raven. Build time for overseers is much quicker than the other two recon units (roughly 17 game seconds), given that you're already forced to graze a stock of overlords afloat for your unit supplies. And despite the ability to maneuver enigmatically and cause damage, it is the only non-structure unit to have no supply costs. Inexpensive, ably-balanced and physically obstinate, these guys are worth the investment in any advanced Zerg strategy.
For scouting purposes, there are several options expendable to you. Overseers deceivingly have great acceleration and movement speed (with upgrade) for its size, making quick peek-and-retreats in dangerous areas, not all that threatening after all. Spawning changelings in enemy bases is great too: these peculiar units morph into a tier one enemy unit; they must be actively detected by your opponent in order to eliminate them. Otherwise they can loiter in the base unnoticed, while giving the information you need. Spawning an infested Terran is more palpable to your enemy than with a changeling, but using them at the right moment can pack damage on enemies and function as a handy diversion.
Infested Terrans are more than scouting tools. With almost identical abilities of a Terran marine unit, their support can make huge differences in a decisive battle. Have your overseers spawn them within or just before the heat of the battle. Before hatching, the eggs of the infested Terrans can tank up damage, giving necessary time and cover for your primary army to attack. They can provide great air and ground support with decent range and fire speed of a Terran marine, but with stronger firepower-a great unit to protect the overseer during a retreat as well. And above all, this extra, versatile firepower doesn't cost supply or resources. If you find your supply maxed out, remember that these hideous ones are here for you.
Feel like being a creep? It pays off to delay enemy construction and researches by using the overseer's contamination ability. The effect of this short-ranged harassment can degenerate your opponent's resources or mitigate their army threat, by halting anything building or researching during the contamination of the structure. It works great after picking off their harvesters and contaminating their main building to prevent rebuilding new workers. Or maybe even contaminating their research will give your army the leverage of the battle: in some cases you can prevent the Protoss colossi from using extended thermal lance technique at the right moment, or delay research of siege mode to flood out your Zergs free games ground units in numbers.
