The difference between a pyramid scheme and a similarly-structured legitimate business is that a pyramid scheme doesn't provide a real product or service that has any value and instead moves around money just by enrolling new people into the scheme who bring the new value in themselves. If bitcoins never take off as a realistic way to transfer value from one place to another and all the value comes from speculation, then bitcoin will eventually collapse for the same reason all pyramid schemes eventually collapse.

It comes down to whether you believe crypto-currencies are a real product and a viable technology for use as a medium of exchange.