Definitely the coolest power up that any of your creatures has. Absolutely the best. The animations are fantastic. There are times, as others have mentioned, that you can't tell what's going on because there are so many damn things happening all around, all at one time. This can get frustrating until you learn to keep your troops spread out a little bit behind you and to your sides.
Sound is fantastic.
Each creature has about 4 or five stock phrases it spits out when issued a command. This isn't overdone and lends a lot of warmth to your swarming, killing horde. I think my favorite voice character was for the Sylphs the archer nymphs of Persephone. They have an airhead kind of delivery that you'll get quite fond of. The enemy wizards swoosh by you cursing and threatening you even when they're getting whupped by your critters.