It's pretty much as awful as you'd expect, laced with the occasional episode that doesn't make you want to smite all of humanity. Fortunately, after the book was published, whites everywhere were forced to confront injustice and rose to the occasion to address these ills and try to stop being assholes. Oh wait, actually Griffin received death threats and was hanged in effigy in his Texas hometown. Way to not be assholes, folks.
A couple observations. Although dark skin brought with it the automatic treatment as an inferior by white society, it also served as an automatic passport to a certain solidarity with other blacks. They knew this poor bastard, fresh off the bus, wasn't going to be able to get a drink of water anywhere in town, much less a place to sleep for the night. In many instances, Griffin gets food and shelter entirely through the kindness of (black) strangers, who have little enough for themselves much less to share with some strange drifter, but know that a cold night's sleep on a roadside is nowhere near the worst thing that could happen to him. There's a touching scene where a family takes him in for the night, and they pool their food, with Griffin slicing his Milky Way bar into thin sections to share with the children for dessert.
Another thing I noted was some sub-sub-subtext in a passage about hitchhiking after dark in one region, and the whites that pick him up ask him all manner of salacious questions, predicated on their 'knowledge' of the black man's anatomical gifts, loose morals and animal appetites. Griffin makes no explicit comment about it, but I get the feeling he found the stretch of road where white guys went cruising for dark meat.
The boy ended up wanting me to expose myself to him, saying he had never seen a Negro naked. I turned mute, indrawn, giving no answer. ... "I wasn't going to do anything to you," he said in a voice lifeless with humiliation. "I'm not a queer or anything."
Sure, maybe it's just clinical curiosity.
For those short on time, here's the TL;DR version of the book, in the form of a 1960 article in Time Magazine.