Mercenaries are used to give your army a bit more *umf* through either raw stats or abilities. Mercenaries can be hired without production-time in exchange of being overpriced compared to normal units. This extra price you pay makes it important to know when to acquire mercenaries and when not to. Hopefully you'll be able to make competent judgement calls on this matter after you've read the following guide!
At Warcraft3.info there will soon be published an article regarding various statistics. The statistics will be based on data from over 400.000(!) games played on the Northrend ladder, extracted by the courtesy of David. So stay tuned for that!
I've gotten the opportunity to use said data a bit in advance, in order to take a look at what races are using mercenaries the most:
NE: 1.4 mercenaries pr. game
HU: 1.1 mercenaries pr. game
UD: 0.6 mercenaries pr game
OR: 0.6 mercenaries pr game
Alright, so Human and Night Elf both hire twice as many mercenaries compared to the other two races. There are two reasons why Orc and Undead hire so few mercenaries:
Meanwhile the same things can't be said for Human and Night Elf as they:
The mercenaries that you are able to buy depends on the tile-set of the maps. There are over 40 different mercenaries in the game spread across different types of map tile-sets. This is, needless to say, way more than I'll be able to cover in this article. Luckily it's far all of them that's seen in 1v1 gameplay. Actually there's only one type of mercenary that is played in competitive matches, which is the one I've decided to cover.
Forest Troll Shadow Priest
Cost: 195 10 2
Having both the ability to cast Heal and Abolish Magic, this is a very versatile unit. In some instances it can be incredibly useful - even game-deciding - due to Abolish Magic. If you are playing against a Human with Water Elemental, a Night Elf with Entangling Roots or an Orc with Spirit Wolves, you'll be able to take away a lot of their early game power with this mercenary.
If you are playing Human mirror, it should be a high priority to creep a Mercenary Camp, due to this unit alone.
Forest Troll Berserker
Cost: 245 30 3
This mercenary have no abilities, but its it's still a very popular because of its raw stats. The fact that there are no abilities to account for makes it very easy to compare the unit to other units. Let's look at the differences between Forest Troll Berserker and Crypt Fiend.
Looking at the resources fore the two units, it's fairly even. Berserker costs a bit more gold in exchange of a tad higher lumber cost for Crypt Fiends.
When looking at their fighting stats it's also quite even. When buying Berserker you sacrifice a bit of your units effective health pool (EHP) in exchange of more damage (DPS).
All-in-all the two units are very even. The comparison doesn't take the two abilities of Crypt Fiends into account nor the instant production time of Berserker, but I'd say it's safe to conclude that it's a quite good mercenary to hire when you need some extra damage.
You should definitely make sure to pick this mercenary up if you are expecting a Huntress push against you as its piercing damage deal a lot of damage against the unarmored Huntresses.
Cost: 145 10 2
Mud Golem does, much like Forest Troll Shadow Priest, not have very impressive stats. Mud Golem can Slow enemy units, so you'll want to pick him up if you find yourself in the need of some extra lock-down for your army.
Cost: 300 4
This lovely brute takes the cake as the least popular mercenary. Though he costs 100 more than a Grunt, he only have 50 hp more than a Grunt with Berserker Strength. He does deal a bit more damage than a non-upgraded Grunt, but that doesn't really make up for his that much higher price.
Ogre Mauler is mostly used as a meat-shield by Humans who's got more gold than they can use due to an early expansion.