It's said that pigs are smarter than dogs. And I can only imagine a tusk-armed, 250 lb, fast and smart hog. No thanks.
They are very smart. Curious and often unafraid of humans too. I know a guy that hunts them with pistols -- so he has to get close -- and more than once he has had a boar follow him and then walk right up to him. The boars usually were not taking a threatening posture, it simply appeared that they found something new in their environment and were thoroughly checking it out.
I like to stake out a position with a clear field of view, but I find myself checking my six regularly, because they will sneak up on you on occasion.