US customs is fine. TSA is a mixed bag. But US immigration are unfathomably rude and disrespectful for no reason at all; they treat you like scum. I've been through US immigration & customs a lot, been sent to secondary three times, ironically the people you deal with in secondary are more polite/reasonable than the people in the little glass booths, but still very "respect my authority."
Ironically going into the US feels more like entering a police state than going into Russia. The Russians were just super disinterested and to-the-point. The US is fine once you're in, but getting in as a non-American is a bad first impression (the UK has this issue too).
I'd hate to think how much worse they would treat me if I was non-white and from a country with traditionally bad or mixed ties to the US. From other people's anecdotes that I've read or spoken to: Bad. Just by what their official union supports (and who) you can get a sense of their views on other races.
Last year the US just started a new policy of asking foreigners for our social media credentials. It is starting out as option, but like all of these things it is just a matter of "when" not "if" it will be made mandatory.