Reading on iPads and Kindles or hurriedly picking up the books in stores, some didn't know about the romance part, thinking the surprise best-sellers by newcomer E L James would be more "American Psycho" than steamy Harlequin.
Others knew exactly what they were getting into, buying into the buzz since the venerable imprint Knopf took on publishing rights, shoring up a story that began as "Twilight" fan fiction and putting it out in handy trade paperback on April 3.
There's flogging and bondage and sex toys. And a steely control freak of a gazillionaire Christian Grey, a damaged sexual "dominant" who enlists the virginal (not for long) college coed Anastasia Steele for rough-but-consensual role play.
Jeremiah Wirth, a grad student and Iraqi war vet in Maine, said the opening book was nothing short of a life-changer. He read it on a business trip to "magical" Hawaii, returning home to Bangor a better man.
"I was away from my girlfriend. I was lonely and I was reading this book in this beautiful place and I thought it would be something fun and easy," said the 26-year-old Kurt Vonnegut and "Star Wars" fan, just a year younger than the fictional Grey.
"People hear about flogging and stuff like that in this book, and they don't get it. I became emotionally invested in the love story, especially from the female's perspective. That was important to me, to put myself in Ana's shoes. It was overwhelming, and I'll never forget it," Wirth said.
He was moved to send James an email, "apologizing for assuming that your book was anything less than it is: wonderful." And she responded, his deep interest surprising even her, "given that you don't fit the demographic of the readership (women 17-100) but I am delighted that you enjoyed it."
The book didn't shatter 66-year-old David Shobin in Smithtown, N.Y. The semi-retired gynecologist and newbie romance reader who writes medical thrillers on the side picked up the first "Fifty Shades" to see for himself "what all the hullabaloo was about."
He liked it well enough and received hundreds of responses to a funny review he wrote on Amazon.
"At my age, my arthritis flared up just reading about Ana's sexual gymnastics," Shobin wrote, adding that her "pyrotechnic climaxes resembled repetitively watching porn: after a while, it leaves me bored and yawning."
He conceded a "definite infectiousness to the plot" but found it hard to believe Ana had absolutely no sexual experience before literally stumbling into Grey's office to interview him for her college newspaper.
"I had an intellectual curiosity," Shobin added in an interview. "I don't quite know what to make of this sort of sexual activity but as a love story, it did succeed."
Will his wife be reading?
"Probably not," Shobin said. "I told her a little bit about the bondage part and she showed very little interest in that, so it was a short conversation. She mainly reads memoirs."
John Puckett, who is gay, spared no superlative from San Dimas, Calif., where he works as a theatrical manager about 20 miles east of downtown Los Angeles. Usually preferring autobiography and true crime stories, he's now reading all three "Fifty Shades" books for a second time.
"I was pretty much hooked from the beginning," said Puckett, 45. "It grabs a hold of you and it doesn't let go."
Most appealing, he said, is Grey's slowly unpeeled vulnerability, that "lost, hurt little boy who craves nothing more than to be deserving of unconditional love."
The books are flying off the shelves at the Books & Books stores in south Florida. James opens her first U.S. tour in Miami on Sunday. Movie rights have already been sold and the guessing game is on over who will play the lusty key characters.
"I first found out about the books back in December, from men who wanted to buy them for their wives," said Mitchell Kaplan, who owns the Florida stores. "You really got the sense that these books are helping relationships in some way."
Dr. Mehmet Oz sees that potential, dedicating a recent show to exploring the books with an audience of women and, yes, men who have read them.
"This woman has gotten people talking about sex in a way that no one else could get them to talk about it," he said Tuesday night from the red carpet of a gala honoring Time magazine's 100 most influential people in the world - James included with the likes of President Barack Obama and Rihanna.
Are sex lives changing, marriages evolving?
"They're not tying up their women. It's not about sadism," Oz said of men drawn to the books. "What it is about is people having an honest conversation about what sex should be like, what makes it feel better, what are the timing issues, how do we make it an important issue in our life rather than an afterthought. When the guys get into it I know we've got something going."
James, a Londoner and former TV producer with two teen sons, didn't attend the event but has called them her "mid-life crisis." She replaced her original Twi-names as her story jumped from free downloads promoted on fan sites to not-free e-books and hard copy from an Australian publisher, then finally Knopf, home to Toni Morrison and John Updike.
Bob James, an ex-Marine and dad of four grown children, has heard of those two, but he's also a regular romance reader and a "Twilight" fan. He first read "Fifty Shades" when it was still fan fiction, coming across it on Facebook and a site for "Twi-moms."
"Most people who criticize it haven't read it," said the 50-year-old James, who is not related to the author. "They take things out of context and just pick the sex scenes out. I liked the romance. Ana is drawing him away from all the bondage stuff."
His wife, a regular volunteer at their church, rolled her eyes when he read her segments, "but not the sex parts," he said. Has he picked up any marital pointers from the attentive yet troubled Grey?
"I learned that I do need to show more of a protective nature toward her in public," said James, in Manassas, Va. "There's something that's drawing women to read it and it would behoove a man to know what that is."