I rented Captain America a while back and admit, it was a lot better than I thought it would be. Certainly a lot better than Sega's godawful Iron Man games.
Arkham-style combat, didn't look that bad, and was a respectable length. Easy to 1000/1000 too.
I don't know if I agree with cjenvy about it being the most underrated comic game, because I still think Xmen Origins: Wolverine holds that title. But this is definitely in second place.