Heart of the Swarm- Evolution Guide: Those Who Glave and Run Away...

Vicious Glave: Your mutalisks’ glaves bounce three additional times, travel further when they bounce, and deal more damage to targets they bounce to.

This is the option for using mutalisks against large numbers of enemy light units.  Striking a total of six targets with each glave can have a huge effect to wear down ground forces or flat-out eat away the lighter enemy units you face, and it can make mutalisks an absolute terror when you bring them around enemy bases into the crystal mining operations.  Unfortunately, the damage still remains poor, but when there are a lot of targets, a lot of mutalisks, or both, this ceases to be a real issue.  Not a good choice if you want mutalisks to be useful for anything other than swarm-stopping, which you unfortunately have a number of other units that do a better job at.

Rapid Regeneration: When your mutalisks have been out of combat for at least five seconds, they heal 10 Life every second.

This is an amazingly useful adaptation for those who use mutalisks as they are best used- as a harassment force, ducking into and then out of combat.  It does little to help them against enemy air units, who can virtually always catch up to them, but it lets them duck into and out of combat too frequently to allow most enemy forces to recover.  This is especially nasty when striking at enemy bases, and micromanagers can abuse it by cycling mutalisks into and out of combat zones to wear a steady hole in enemy defenses.  Sadly, this doesn’t keep mutalisks from dying in combat at all, but then they were never meant to stick around in protracted fights to begin with.

Sundering Glave: Your mutalisks’ glaves deal double damage versus Armored units, but do not bounce anymore.

This adaptation is very questionable.  While the doubled damage agaisnt armored targets is an excellent way to take out heavy assault units that can’t reach the mutalisks anyways, such as siege tanks, colossi, and ultralisks, the loss of the bounce effect takes away one of their main selling points as harassers- the ability to weaken a number of enemy units at once and soften them up for the kill.  This would be a much more sensible adaptation if mutalisks lasted long in combat, but they remain frail.  That said, it can be hideously effective at allowing you to inexpensively take out individual powerful enemy units, and can be especially fun if you then lure enemy anti-air forces into a trap with your fleeing band of badly injured mutas.  This basically comes down to how good you are at having your mutalisks duck into and out of battle.