As a Zerg user myself, I have to disagree with mfk. I will counter his points thus!

"Their HP is simply too low. With a pitiful HP of 35, a group of Siege Tanks - even any unit that can inflict a decent amount of damage - will make short work of a group of Zerglings."

On the other hand, you get two Zerglings for 50 minerals as opposed to a single 40 HP Marine for 50 Minerals or a 100 HP & 60 Shield Zealot for 100 Minerals. Four Zerglings, in the case of a Zealot, would have a collective 140 HP and would deal a collective 20 damage. And when you upgrade, they get even more powerful. Sure, a cluster of Siege Tanks (which isn't realistic in a Zerg versus Terran fight, you usually see Marines and Medics with one or two tanks and perhaps a Science Vessel assuming the match is in the mid-game) can blast them to death, but they cost so little that they are a cheap distraction from your real attack force, such as a group of Lurkers advancing from behind (Lurker-Ling is quite common in Zerg vs Terran) or Mutalisks dive-bombing from the skies (Zerglings take hits while Mutalisks, normally eaten to death by the bullets of Marines supported by Medics, fire death from above).

Against another Zerg opponent, pretty much the ONLY attack forces you will see are Muta-Ling or Scourge-Ling (Scourge-Ling as an alternative to Muta-Ling vs Muta-Ling, of course). Why? Hydralisks deal explosive damage, so they get eaten by Zerglings and Mutalisks which are both normal damage dealing Small units. The Hydralisks deal 5 damage because explosive damage is cut in half when the target is a Small one (the logic being that something small can't feel the full brunt of the blast) while the zerglings deal six damage and the Mutalisks deal 9, 4 and 1 with their bouncing attack.

Against Protoss, the Zergling isn't that common except in combination with other units (in fact, it's rare nowadays to see a player produce a unit of only one type) or when a player is invoking the Sauron Style: a strategy in which the player mass produced Zerglings in order to overwhelm the opponent.

"There are better units to spend larvae on. The Hydralisk, which is by and far the most overused unit in the history of Starcraft, provides all the damage the Zergling does with an ability to attack air units and to morph into Lurkers. They are somewhat more expensive than Zerglings, but 20 minutes into the game, a Cerebrate should be able to produce Hydralisks without depleting resources."

The Hydralisk is indeed versatile, but the game has changed. Hydralisks are used pretty often against Protoss, not too often against Terran (because Marines and Medics can fight them off with ease) and rarely against Zerg (because Muta-ling eats them up).

Also, you don't often see games go to the 20 minute mark without players annihilating their opponent's expansions multiple times over on higher levels of play (on lower levels, both players tend to turtle up and not expand).

"Zerglings have a melee attack In order for the Zergling to do any damage, it must first run up to the unit before attacking. The vast majority of Terran and Protoss units have distance attacks which will destroy the Zergling well before it reaches its intended target"

Zerglings with Metabolic Boost can run up to the enemy quickly. Zerglings that are flanked properly (that is, they don't rush toward the enemy in a steady stream like many newbies command them to, instead they attack from multiple sides in staggered intervals) don't fall victim as quickly to ranged attacks. Zerglings that are burrowed lose the melee disadvantage completely and gain the element of surprise. Zerglings under a Dark Swarm cloud are immune to ranged attacks. Zerglings paired with Ultralisks don't die as quickly because the Ultralisks take more of the damage than the Zerglings.

"They are difficult to manage. A large group of Zerglings scattered over the Creep can make for interesting times; just try rounding them up into Overlords."

No objections here! I try to organize my Rally Points and hotkey my attack groups so I don't run into this problem as much as I used to.

So, you see, what mfk has written on zerglings is simply not accurate. The highest-ranked Korean Brood War player in the world is currently JulyZerg, who uses Zerglings quite often and with good reason.