It's not that they hadn't played Sonic - just that it wasn't the first to get them hooked. I think most of the middle-aged or slightly younger guys/gals here probably played Sonic back in the day.
Of course, I said that referring to the thread's title. Saying that Mario Bros. was addicting and not referring Sonic is, atleast here in Europe, a blasphemy (Sega was A LOT more famous in Europe than Nintendo back then). But, since the forum is American, as well as most of the members in the topic, I kinda understand. Altough Sega gained significant market in the US as well. But I'm going offtopic now.I love sonic, but it was not my first game.
Probably Tomb Raider followed by Homeworld.