I flew to Oshkosh last year (July), arriving in the USA via Canada (obviously).
The first question I was asked by Customs & Border Protection (CBP) was "Did I have a Visa?". Had I not had one the fine would have been somewhere in the region of $4500.
So - notwithstanding all those who "think" it's possible, it all boils down to - are you prepared to take the risk?
A little bit of form filling, a little bit of money (relatively speaking) and a day out visiting the embassy works out a lot cheaper in the long run.