There are two important milestones of assimilation for a Briton living in the United States, according to Johnson, The Economist's language blog. One is to start calling the final letter of the alphabet "zee" instead of "zed." The other is to start responding to "How are you?" not with "I'm well," but with "I'm good."
I'm not so sure. As an American in Toronto a few years ago, I found the zee/zed transition in the other direction surprisingly hard, but I felt I had to make it because so many people seemed simply not to understand "zee." Surely "zed" would be even more baffling to Americans. So I think zee-for-zed would be a conscious decision early on rather than a sign of having finally gone native.
And I'd agree that any Brit who said "I'm good" instead of "I'm well" probably was pretty far down the path to Americanization. But is "I'm good" really a universal Americanism? Please, someone, tell me it's not. After all, you don't really have to say, "I'm well," if that sounds just too upmarket for you. There's always "Fine, thanks," for heaven's sake. Oh, and will you please hand me the sugar tongs when you're through with them
The blogger Grammar Girl is often on the side of the (prescriptivist) angels in her advice, leading her readers through the wilds of lie versus lay, for example, and illustrating her points with quotes from lyrics of songs by Bob Dylan and Eric Clapton. But she came out a few years ago with a stout defense of "I'm good" as a response to "How are you?"
"[T]here are grammar nitpickers out there who will chide you if you say it. The wonderful news is that those nitpickers are wrong: it's perfectly acceptable to say, 'I'm good.' "
But here's why "I'm good" drives those of us in the rear guard up the wall: Although it's true that well is the adverb corresponding to the adjective good, in the sentence "I am well," well is an adjective meaning "in good health."
"How are you?" is a polite ritual inquiry, and it's a greeting, not a question, with a fairly narrow range of acceptable answers, from "I am well," to the aforementioned "Fine, thanks," to "Can't complain," which hints that one might like to complain but is aware that politeness precludes that. Someone being asked, "How are you?" is not expected to give a Zagat or Yelp rating of his or her performance in life. ("I'm terrific – five out of five possible stars!")
Well is under pressure from good in other contexts as well. A job that pays well is a well-paying, not a good-paying, job, but that rule seems to be honored more in the breach than the observance of late: A quick Google News check has just shown 522 hits for "good-paying jobs" but only 267 for "well-paying jobs."
There's a variant of "I'm good" or "I'm OK" that is part of the language of informal negotiation. "Are you OK if I get it to you Monday?" "Yes, I'm good with that."
Then there's the "I'm good" that means, "in no further need of anything from one's server in a restaurant."
"Are you all set?" she may inquire. "More coffee?" "No, I'm good."
I'm OK with that. But I'm still not OK with the more general "I'm good."