Insanely valuable. 2 free allies for the cost of 6. If you play it right you could get up to 12 resources of allies. (6 is the highest cost allies) Imagine playing spider-woman and easily getting both Hercules and Captain America out.

So without playing Maria hill. This hellicarrier is awesome.

Without Maria Hill, it's hard for this card to be worth it. Captain America and Hercules are fine and all, but they're not really developed assuming you're going to pay 6 for them. Heimdall is probably a more realistic cost for the best ally you'll play with this.

So, you've played 7 effective resources for the Circe, and each hero is another effective resource, bringing the total to 9. Playing Heimdal and Giant Man from your hand would cost 12. You've likely spent an entire turn just playing Circe, and then you have to wait to draw your giant allies, and can play one a turn. Also, if you need to be putting out fires, you can't afford to spend 6 on this, and you are drawing these giant allies who are of limited use in most decks.

I'm not saying it's useless, but it definitely shouldn't just be thrown willy-nilly into decks. You need to consider the costs/risks.