Illusionary Mask is pretty unique. Well, not as unique as it was - until morph came along, it was the only source of face-down critters. Now with morph, megamorph, manifest, cloak, disguise... well, there's plenty of it flying around. But in addition to being the first, the Mask also has a few unique features! It lets you play any critter facedown and it also lets you flip them up for free - all you need do is attack!
Now, you do have to pay the creature's original cost, which means you can't really cheat out any goliaths. But even if you do pay the mana cost (and remember, you have to pay the right colours, too) what you don't need to worry about are "additional costs" or unwanted comes-into-play abilities. Thus, a Guardian of the Great Door or Bayou Groff can be slapped down for two mana, and... you're good! Next time they swing, you have yourself a cheap powerhouse on the prowl. Nice!
Paying a creature's cost and flipping it for free also lets you save some mana on actual morph creatures, funnily enough. Ainok Survivalist can be played facedown for , then next time it attacks or blocks, you can take out an artifact or enchantment for free! Master of Pearls saves you a whopping six mana, providing a mini-Overrun when it goes to attack. Just don't forget that the boys with megamorph won't be picking up their +1/+1 counter if you flip them this way - but of course, you can also just pay their morph costs if you'd rather!
