I disagree. It's basically a third rate aggro paladin deck until turn 6, and then mysterious challenger is only like, super super strong if the paladin has board control. Guess what other deck is guarantee to win if it has board control at turn six? That right, pretty much any other aggro deck. It's a step above aggro paladin for sure, since with some lucky card draws it can punch a bit into the late game. And it's a lot stronger vs heavy control decks that don't try to contest the board at all and instead rely on removal and board control. But I feel like it could actually be weaker against mid game decks, which has less of a chance to run over in the early game then just normal aggro paladin and will actually be ready to handle the mysterious challenge when it's issued. And since mid range got wrecked by patron warrior even more then aggro, their reintroduction to the meta I certainly feel could have a tempering effect on secret paladin.
I mean, of course that could all be bollocks, but that's just my experience with them, as a person who primarily plays a midrange style, they've always felt like they give up just a bit too much of the power in their deck to make their turn sixes so good.
Edit: That's not to say it won't be an aggro meta for a little bit at least. Decks like face hunter and mysterious challenger always get the best showing at the start of a new meta when the decks all suck. But in a few months when people are more used to it, I'll just go out and a limb and predict that secretdin has a fall. If not from tier one, at least from a assured first spot (where it looks to be for now at least)