Bitcoin is a payment network. To make a payment using Bitcoin, you buy some bitcoins on an exchange, then you send them to the seller, who sells them on an exchange. Where is the scam in all this? You paid your $x, the seller got his $x. That's not a scam, that's mission accomplished.
Not really. A viable payment system lets you get real money in exchange for something; with Bitcoin there is no assurance you will be able to get anything beyond some bits and bytes. You are at the mercy of the exchange and if they don't have the cash you don't get paid. One huge red flag is the seeming arbitrage opportunities with Bitcoin process varying exchgane to exchange. If Bitcoin were a viable transaction system those opportunities would disappear as people took advantage of the free money. That they don't says a lot about the liquidity, or rather lack of it, in the Bitcoin market. A currency that is touted as being easy to use for transactions anywhere with no transaction costs would quickly erase any arbitrage if it really was that easy to buy and sell.
All this noise about Ponzi Scheme: Yes or No? masks the real issues with Bitcoin. It is an illiquid, volatile commodity that lacks any assurance you will ever be able to get real money for it; as a result almost no one really takes Bitcoin, they simply let you "Pay" in Bitcoin by assuring they can immediately convert them to real money. While that may work for a handful of deals the lack of liquidity makes them unusable as a real payment system to take on PayPal or other electronic payment systems. Unlike Paypal, which mostly just moves money for a fee or holds it in a form that is easily turned back to cash as needed; Bitcoin "exchanges" take short positions that they can't cover since they lack the cash reserves to payout all the withdrawals.
As a result, Bitcoin became a nice place for speculators and people who are hoping to cash in on the "next big thing" and now the bubble is starting to burst. I wouldn't say it is a scam as much as one more chapter in the mass hysteria of crowds.