If you are looking for a fast air attacker, then the corsair is your choice. For 150 minerals and 100 gas, a decent health, shields, a quick hitter neutron flare for 5 damage, and a speed overlords cannot flee from, then the corsair is you bet. Against the zerg, corsairs are used to slay overlords, disruption web sunkens and fend of hordes of mutalisks. They are fairly effective in handling all these and with the splash damage, mutalisks will fall before armies of corsairs.
Terran defense with all those bunkers, siege tanks and turrets almost impossible to break through. Corsairs disruption will give you a better chance. Combined with dragoons, these air stirkers will become support to a very dangerous attack force.

• Disruption web nullifies all attack coming from the ground units it covers, use it effectively.
• Upgrade d. web in the fleet beacon right away.
• Damage upgrades for fliers are good if you plan mass corsairs.
• Construct 2 to 3 stargates for quick production, when you have about 3 corsairs, you can start overlord hunting.
• Build shield batteries so that the corsairs can run and recharge and then strike again.
• Use corsairs to support your reavers and or carriers. Let the reavers and carriers do the heavy damage and use the corsairs to protect the heavy firepower from critters.
• Zealots are also good to combine with corsairs because both can handle most types of defense and attack.