Polukranos, World Eater is a cool card. Stompy hydra, efficient stats, interesting ability. The monstrosity cost is pretty steep, so you usually won't do much more than pick off a few mana dorks... but if you have infinite mana, it's a
Plague Wind, which is sweet.
In a vacuum, I don't think Polukranos is amazing - a 5/5 for four mana is certainly above the curve, but that isn't quite as valuable in this format. Simultaneously, the activated ability is pretty expensive to use, and it's also vulnerable to removal - if you kill a creature in response to its monstrosity ability, the 'when it becomes monstrous' trigger never occurs. I've blown out a lot of Polukranoses (Polukrani?) and
Hydra Broodmasters.
Still, if you're doing a stompy deck (such as
Goreclaw, Terror of Qal Sisma or
Ghalta, Primal Hunger, Polukranos seems like a reasonable include. Also a good inclusion in
Gargos, Vicious Watcher hydra decks. Crucially, this style of creature tends to result in really fun gameplay - smashing with beefy hydras is always very satisfying to both do and watch.