When is a shoe more than just a shoe? When it’s by Manolo Blahnik.

Just ask Madonna, who says Blahnik’s shoes are “as good as sex – and they last longer.”

Or fan Sarah Jessica Parker, whose “Sex and the City” alter ego Carrie Bradshaw shares the actress’ obsession with Blahnik’s shoes.

Or Bianca Jagger, Diana Ross, Gwyneth Paltrow, Raquel Welch and Kate Moss – all of whom go weak in the knees for high-heeled, high-dollar Manolos (or “Blahniks”; fans refer to his shoes by one name or the other, but rarely both).

Blahnik, who reportedly can’t get the word Birkenstock to pass his lips, was born to a privileged family in the Canary Islands, though he’s now based in London. He began making his shoes 30 years ago and now caters to an international clientele of devoted collectors.

The designer, who is in his late 50s, dreams up 300 new glorious pairs a year, each of which are handmade in one of four Italian factories (a dozen or so workers labor on each pair).

They come in lush satins, silks, brocade, lace, suede and fur. They are often covered in sequins, beads, pearls and – for those willing to shell out a few thousand bucks more than a pair of basic Blahnik’s normal $350-and-up asking price – even rubies, diamonds and emeralds (oh my!).

Blahnik’s life story and a selection of photos and sketches has been collected in a lovely new book by author Colin McDowell, titled “Manolo Blahnik” (HarperCollins).

His fans think this honor could not have been given to a more deserving designer.

“There’s nothing that makes you feel sexier,” Allure magazine editor Linda Wells says of Blahnik’s shoes. “I swear, they make you lose 10 pounds! You walk better in them. And they’re totally comfortable. I once told my husband that they’re like bathroom slippers, and he sort of gasped.”

Reconsidering, Wells admits that “comfortable” is maybe not the best word for the shoes, many of which sport 3 to 4 inch heels.

“If it were just comfort, we’d all be walking around in loafers all the time. But compared with some other pairs of shoes – the ones that hurt when you (MD+IT)sit(MD-IT) – they are comfortable.”

Joan Rivers, who fondly calls Blahnik’s creations “slut shoes,” seconds Wells’ emotion: The comedienne professes to exercise in a pair of his flats.

Another testimony to Manolos’ great fit come from Alex Marshall, executive editor of EdificeRex.com, a Web site on lifestyle issues.

“I’m a total pro at walking in heels, but when I first put them on, I could tell their balance was different, really good,” she says. “And the leather is comfortable and there was no breaking-in thing.”

Marshall, who is 30, got her first pair – black, 3½-inch skinny heels – on sale at Barneys New York about three years ago. “Even though they were on sale for $250, it was a big deal,” she says.

“Buying them was definitely a rite of passage,” she continues. “They’re really serious shoes, but they’re also the most fashionable shoes you can buy. You have to be a little older to appreciate them.”

Wells agrees. “Like any item of luxury, they should be something you look forward to,” she says. “They are not shoes that should be taken for granted. You want to appreciate them. They should be something you earn.”

She once practiced what she now preaches.

Wells got her first pair of simple black Manolo pumps in 1985, “when I’d just gotten my job at the New York Times and I was feeling flush.” Still, Wells wasn’t making much money, so to afford her purchase she scrimped. “I took the subway to work to and from my cruddy apartment and I ate Ramen for dinner and yogurt for lunch.”

Trang Chuong, research editor at Marie Claire magazine, got her first pair for free at one of the magazine’s fashion department giveaways. Two years later, Chuong defines herself as a shoes fanatic so prone to blowing a huge chunk of change on shoes that she brings someone along when she hits shoe sample sales to act as her “bank.”

“My friend was there to control me at the last Manolo Blahnik sample sale I went to,” Chuong explains, “so that I wouldn’t buy more than my limit: two pairs, max.”

Predictably, after Chuong narrowed her choices to three pairs, she begged her friend to be allowed to buy them all.

“She wouldn’t let me get the third pair, so I went behind her back and bought them,” Chuong admits. “When she found out, she said (-ssq-)You’re worse than an addict! I’m trying to help you and you won’t let me!’

“That was the last time I went to a Manolo sale.”

Blahniks also last.

Chuong assumed her favorite pair of lilac slingbacks (the famous first free pair) were ruined after they were doused with a huge pan of oyster water. But they cleaned right up.

“They don’t fall apart,” explains Stacey Wien, a 22-year-old Web site developer who was given her first pair (black boots, skinny heels) by her mom two years ago. “And if they do, you can take them back to the store and they’ll fix them.”

Jennifer Pucci, 26, a public relations executive, is thrilled to have recently purchased her first pair of Blahniks.

“My grandmother decided that it was time for me to graduate to a more refined heel than what I had been used to,” says Pucci, who also wears Prada and Charles David. “She told me, (-ssq-)To be successful in New York, you need a good pair of heels.'”

At first, Pucci admitted she thought the – shoes black pumps with a wrap-around heel straps – were too expensive. (just under $500).

“I told my grandmother that [the price] seemed like an awful lot to spend on a pair of plain black shoes,” she says. “But then I put them on, and walked as well as I could to the mirror. When I saw them on my feet, I got it.”

“These aren’t just plain black shoes. They’re Manolos!”