Azure Dragon:Azure Dragons... They are so rare and they cost so much that my experience with them is rather limited. They are virtually immortal, superfast and random enemy creature stacks are paralyzed each turn. (averagely two per round) If you could ever buy them, then do not let the opportunity slip away!
Boar:Boars are rather boring creatures with average statistics and cost.
Crystal Dragon:Crystal dragons have one great drawback when compared to the other neutral dragons, they can't fly. They deal very much damage, are fast and tough plus they produce crystal. I prefer other neutral dragons though, they are usually better in combat and cost less.
Diamond Golem:Diamond Golems are not that useful, unless you already have stone/iron golems and you are able to upgrade them or if you want to use the Diamond Golems in a combo. (see the 'Spells section')
Enchanter:Enchanters are great. They cast group spells and fire deadly bolts of energy. The problem with Enchanters is their huge cost in gold. Unless you have a good supply of them (like an Enchanter's hollow) or a large stack wants to join your army for a price, don't buy them. It is also good to know that when you split a stack of Enchanters, they will cast a great number of group spells in the same time.
Faerie Dragon:Though their cost is incredibly high, in fact much too high, (as with all neutral dragons) Fearie Dragons are very useful in ANY army. Their speed and abilities along with their massive amount of health allows them to survive longer than any other creature (other than neutral dragons) would.
Gold Golem:To Gold Golems you can apply the same rules as to Diamond Golems. (Gold Golems are worse)
Halfling:Halflings are the weakest ranged attackers in the game, but their great amount of shots and their greater damage range makes them equal to Master Gremlins. In the early game these creatures can be valuable, especially in larger groups.
Mummy:Mummies are below average creatures, plus they are undead too. (Morale -1) They are not worth buying unless maybe you already have an undead army, and even then...
Nomad:Nomads are above average creatures with a nice special ability. The only drawback is their relatively wide damage range.
Peasant:The most useless creature in the game (ever) is only interesting for a hero with the Necromancy secondary skill and even then only to kill it.
Rogue:Never underestimate Rogues, they are able to compete with Halberdiers in combat. Their cost in gold however makes them less interesting creatures to purchase.
Rust Dragon:Rust Dragons cost even more than Faerie Dragons, but their force and ability is (almost?) unmatched. The Rust Dragon's acid breath lowers the target's Defense skill by 3 every time and sometimes it deals 50 additional acid damage (per Rust Dragon in that stack). As the target gets weaker, (less Defense skill) the attacks of all your creatures against that target become more and more effective. Rust Dragons are true killers but they are very rare. (all the neutral dragons are)
Sharpshooter:Sharpshooters have the same problem as Enchanters, they cost too much. Still, if you could ever recruit large amounts of these, be sure to do so. Sharpshooters are among the best ranged attacking creatures in the game.
Troll:At first sight Trolls would seem rather low quality creatures, but their regenerative ability surely makes them a worthy foe.