It would best be described as a pyramid scheme, where as a Ponzi scheme has a single 'master mind' at the top who takes in and gives out all monies (and who will inevitably try to abscond with it all).  A pyramid scheme is decentralized, so when people say "BTC is not a Ponzi because it is decentralized" they are correct, but this dose not mean it is not a scam, decentralized scams were created LONG before the internet.  

A Pyramid scheme essentially ranks it's members by how early they were involved and new members must pay old members for joining.  In a Pyramid scheme the earlier adopters often have a finite payout they receive when a quota of members below them is reached, essentially the Pyramid fissions at this point with the second tier individuals now becoming the leaders of their own pyramids.  It is a scam because it can only work if grows exponentially forever and this is impossible in a finite population, the last people to join are the ones screwed over.

BTC modifies this by never mandating a fission of the pyramid, rather it allows early adopters to keep riding and hoping for bigger rewards for as long as they can delay their gratification.  Also it allows a degree of reshuffling of ones hierarchy in the pyramid by continually buying more coins.  But the basic nature of the pyramid is still their, all wealth is merely being redistributed (upwards always upwards) rather then actually generated by real business activity.  The key to making it an effective pyramid scheme was the nature of mining and it's exponential decreasing effectiveness which put huge quantities of coins in the hands of early adopters while guaranteeing future scarcity, thus creating the incentive for them to hunt for more participant, thus elevating themselves.

BTC thus combines the elements of a Pyramid scheme with a speculative Bubble, and it encourages each aspect to feed on and accelerate the growth of the other.