Turn 4 hastey uncounterable Grozoth? Yes please! Really, I'm surprised this card doesn't see more play, especially in formats like EDH. Yeah, if the creature you put into play doesn't make it until the end of turn this doesn't come back, but the card you're putting into play probably requires a large amount of effort to get rid of. If this thing gets targeted, suddenly it's a Petradon and makes that opponent sad. If your opponent has no blockers, it becomes Living Hive. Being attacked? Here's an Engulfing Slagwurm. Not to mention the benefits of cheap instant-speed Progenitus. Still, there is the issue of potentially losing this card permanently. That, and the card itself is only a 1/1 for 3. Still, I think it's too underrated.
Pros: Cheats in creatures with haste at instant-speed
Cons: Creature doesn't stay around, card can get exiled