Depends.
I like Sonadow a lot but this pair is mostly used for porn along with another pairings.

Fan-story wise, I would love to see reasons behind Sonic and Shadow being a couple - I never read the story with the explanation - That's a example, mind you - "(..)It begin when Sonic saw Shadow on the ark.." - exscuse me, what? They didn't know each other and they were enemies back then! (besides it set the mood for the rest of the story, you kow?).And I don't suport rape!SS pairig (I'm no-porn person.). Hoever the AU setting is fine with me as long as the criterias above are fullfied.

However, in the reality, I'm fully aware thet the bond between Shadow and Sonic would be rival/frenemy, or (huuge enxtension needed here  or next game) brother to brother relationship.