I personally like them as well. They are kind of hard to keep alive in the early stages of the game (where you need no-loss victories and the heroes drop flaming arrows on them regularly), but in any case, are extraordinary useful. Their mass attack generates a ton of rage and they can summon meatshields. They hurt a little the morale of all human troops, but it is manageable.
Other units I sorely underestimated were:
Druids - meatshields, mass attack = ton of rage, animal control, decent HP
Black Knights - battle winners with Splinter of Darkness, even if you play with 1 stack
Choshas - meatshields every 2nd turn that hit everywhere on the battlefield
Gorguanas - 2x damage is simply godly
Both types of imps - they are great support against slow units (speed 2 or less). Their strike and return ability is great, as it gives you a free hit without having to use target all the time, which is the main problem for an arny with fairies.
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