That's actually pretty true, it will help mainstream adoption.. but at the same time, it's not good if one site has all control in the say so of price rates to buy bitcoin... especially if the government is affiliated with them.
If their prices veer away from any other market player then people will take their business elsewhere.
How big is Circle at present? I don't see them discussed all that much on here.
Well it's pretty hard to do that if you live in the US and don't live in any major US city. Hardly none of the US cities have bitcoin ATM's and localbitcoins usually only work for people in large cities that have atleast some people that use bitcoins. Also, Xapo is another way to go about getting them, but they charge a pretty high fee if I'm not mistaken.
Circle I think is a pretty big player in this field right now, especially since they are backed up now by big time investors.