Know Your Zerg - Spotlight on Mutalisks

Here is some key information on Mutalisks.  All information relevant as of patch 1.3:

Are Surprisingly Good Against:

• Void Rays:  equal numbers of mutas will kill a nearly equal numbers of Void Rays.  Void Rays cannot escape Mutas, and are more expensive.

 Are Countered By:

• They cannot outrun Phoenixes - stay and fight unless you can find nearby corruptors or other anti-air to save them
• Are shredded by most anti-air, but especially low damage, high-rate attacks such as Marines or Hydras

Best Purposes

• Should be constantly on the move, preferably harassing something:
• Workers
• Enemy Reinforcements
• Slow enemy units (Siege tanks, Immortals, Banelings

Upgrades

• Attack upgrades are better for them, since you will generally be using them to attack things that can't fight back, or that need to die fast, such as turrets.
• If you intend to make the "big Muta ball", armor upgrades are actually quite good.  Still, mutas should not linger when they are being target-fired by enemy units.

The Magic Box

• The magic box technique , conceived by Team Liquid, is a positioning tactic where you mutas are spread out and not overlapping.  This allows them to take on even scary units like a Thor or two without taking massive splash damage.
• To magic box, give the mutas a move command to a safe area, and then wait until they all arrive at the spot - they will naturally spread out.
• When attacking with your magic box, do not target-fire units.  If you do, the mutas will lose their magic box and clump up.  Instead, use a move command and park your boxed mutas over your target, using the Stop command when your mutas are in position.  Mutas have a bounce-attack and will hit what you want them to most of the time, since their range is so short.
• Watch a video tutorial on the magic box and its benefits