Whatever happens to Circle, just remember this:
If you decide to use them to buy bitcoins, always immediately withdraw those bitcoins to a real wallet.
Same thing applies to any other service of this kind.
Exactly. frank1 got it right too.
They set up circle to operate as a "bitcoin bank." And what do banks do? Play with your money. Once everyone starts using this as a quick buy service and never let their coins sit, Circle will crumble. It will happen gradually. First, the instant buy thing will stop. Then they will start trying to sweeten the pot for people who keep their BTC in Circle wallets. Then, more fees will be introduced. The customer service will begin to suffer.
Watch.