Marvel War of Heroes is a relatively new trading card game so it didn't surprise me when they released a pretty nice referral card, it always helps getting people in the door. Over time as cards tune up I'm sure we'll find Secret Agent Black Widow will turn out to be weak in comparison, but as far as new decks go, she's solid.
She is the only card at the moment that requires four cards to hit final form making her more expensive to max than normal rare cards though.
Her final form stats are around 6500 atk/def and she comes with a 9% boost to all cards attack and defense. Although the boost doesn't seem great, it's really solid in comparison to most easily accessible cards.
When compared with Rare Nova evolved into SR, she has 400 more attack and a better buff. Although she is still lower than true SR cards.
The nice part is with the game being so new, there are a lot of people out there to join that need to be referred. I would highly recommend players new to Marvel War of Heroes try to gather a deck of Secret Agent Black Widow.