I'd met a friendly border guard once... though he was on holiday at the time.
But on a serious note, I've not traveled to the US much yet have not had any problems... yet. For the most part, I treated the border guards with respect and they treated me in kind. Of course I'd always done my homework in ensuring I had whatever documents i'd needed and had the documents ready the moment it was my turn.
I'd never had a problem yet, and each time, I've noticed that they do have computer access and so would assume that in any case of doubt concerning this or that law, they could consult their database quickly enough for a confirmation.
Also like any other human, I can't imagine most border guards wanting to deny someone entry knowing that it's only going to cause hardship for both parties anyway. The border guard wants as smooth and easy a day for himself as possible before going home int he evening and enjoying a good dinner and some TV. He doesn't want to have to deal with causing irritation unnecessarily as that means more stress for him. My guess is,a few bad apples aside, a border guard will never deny entry unless there is a very valid legal reason for doing so, and will also try to diffuse any tension as quickly as possible by offering any reasonable assistance to help the person affected, such as giving him the address of the nearest US consulate or embassy, informing him of what documentation is lacking, and showing some empathy. He just wants to get that irate traveler out of his site ASAP, and making him more irate will just mean more irritation for himself on a nice quiet day at work.
So my guess is, bad apples aside, most border guards will not deny entry unless they have no choice in the matter.