Paying 1 BTC now to get 0.5-.6 BTC later is a losing move. It doesn't matter what bitcoin does, it can go up, it can go down, it can stay exactly the same as it is, you willingly, of your own free will, gave them money now, to get less than that back at some point in the future. It doesn't have to be a Ponzi scheme, you lost money on this deal either way.