Spidey
Yea but it takes a total investment of 9 mana before you can wax the first critter....9 mana. That's a lot bro.
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Of course there's always exceptions, however when stacked up against creature removal currently running in Type 2, it just doesn't add up well and Type 1 and Extended..let's not even go there. The only two colors which might think about running this, blue and green really have better alternatives in their respective fields of expertise. If I was running mono green I'd much rather play a 6-8 CC fatty then this. In a case where I was rolling with blue, Fatty Flier, Bounce and Creature stealing are more appealing possibilities.
Here's an example:
If your deck contains White, Black or Red your not even going to consider this card regardless of format. Why pay 9 when Terminate, StP, the Edicts, Lightning Bolt, Exile, Path of Peace and so so many more are available?
Now if your Blue or Green Heartseeker might be a little appealing. Both are considered weak in the creature control department so Heartseeker just might make it.
Now if you’re blue you've got a world of efficient control cards at your disposal. Control Magic, Treachery, Mana Drain, Counterspell. Tons of answers. While you can't right kill it, you CAN deal with it in some shape or form.
Green however is a little more difficult. Mono green is traditionally some type of pound you in the face horde deck, either with midgets or fatties. So Heartseeker doesn't make too much sense here either. Your win condition is putting creatures on the table, so your mana spending priorities are creatures..not creature control.
Or if your running some crazy mono blue/green combo deck then your entire pile of cards are tuned to getting out that combo...not spending 9 to plink off one creature.
Come on guys let's be real. Faced with all the other alternatives us casual players have would you really run this thing? Again there's going to be exceptions but those cases are few and far between.
This is another example of a really cool idea gone awry by WoTC's sheepish nature. They're so scarred of creating a "potentially" broken card that they'll instead create a poopie card which won't see play.