This is a very underrated show, doesn't get the credit it deserves, so it's like a cult it, which is kinda cool cuz then it makes the fans part of a secret club or something lol kidding. Anyway, they're very famous among fans, obviously. Also, because it's filmed in Canada, Jensen and Jared don't get hounded by all the paparazzi like in America, so they don't get all the widespread attention like on Gossip Girl, even though Supernatural clearly has more substance than GG and the acting/writing is superior.
I'd have to say Jared Padalecki is more famous though because he was on Gilmore Girls. Jensen Ackles has some attention from Smallville, Dawson's Creek, etc, but he's probably not AS famous, which I think they like, honestly. They have more privacy that way.