THEY say it takes about three weeks to recover from a SWS party, which is generally an impressive recommendation for a fun night out.
The fact that those who've been are always champing at the bit to go back is another sign that a good time is being had by all.
Clearly this is not your average Welsh ex-pat society where members have to be rounded up for an evening of cawl and Cerdd Dant at the local three star hotel every St David's Day.
Founder Stifyn Parri shudders at the very thought. He's worked hard over the past decade to cultivate the sex appeal of being Welsh and he rightly takes a modest pride in what he has achieved.
The SWS - Welsh for "kiss" - membership register, which now has 5,000 names, reads like a Welsh Who's Who for the world of glamour and celebrity, to the extent that it's practically a cliche of itself.
Top of the bill and a club patron is, naturally, Catherine Zeta Jones. It is her lips, legend has it, on the club's logo. The other patrons are also among the Welsh finest - venerated actress Sian Phillips, opera singer Bryn Terfel, Olympic athlete Colin Jackson and pop star Bonnie Tyler.
And no prizes for guessing who some of the other prominent members are - Ioan Gruffudd, Mathew Rhys, Sian Lloyd, Katherine Jenkins, Rhys Ifans, Ffion Hague etc, etc, etc.
So you'd be forgiven for assuming SWS - Social, Welsh and Sexy - is just a club for Welsh luvvies. Well, think again, says Stifyn. SWS, he insists, is a club "exclusive to everyone".
"Yes, people tend to see SWS as the Sian Lloyd and Stifyn Parri champagne club," he admits. "We have got a lot of celebrity members and, to start with the club was an extension of my social life. But it really is a club for everyone - members come from all walks of life. You don't have to be sexy to join or rich - membership is only a tenner a year. You just have to be Welsh, or enjoy spending time with Welsh people, and be sociable.
"When I was a child I always seemed to be excluded from things - either I was too fat, too camp, too Welsh-speaking, had the wrong Welsh accent or went to the wrong school.
"Everyone's face fits at SWS. I want people to come, have a good time and feel confident in their Welshness and not apologise for it."
So, relax, you don't have to reach for your fishnets and spiky heels for a SWS night out. But be warned - such is the reputation SWS has built up over the past 10 years, being a member could considerably boost your sex appeal. Its confident, sassy parties, always held in the coolest clubs, are regarded as the place for ex-pats to see and be seen in as well as have a fun night out.
The opening of the New York branch on the club's fifth birthday, for example, attracted the attention of society magazine Tatler, no less.
The founding members may not be as fresh-faced as they were a decade ago, perhaps, but there's no shortage of new blood knocking eagerly on the door - Katherine Jenkins, for example, who was only 15 when the club held its first event but now holds on proudly to her membership card.
Yet the secret of SWS' success says Stifyn - and try saying that after one of its parties - is apparently nothing to do with being sexy. That was a red herring all along to attract everyone's attention. Rather, the bottom line has always been about meeting people and having fun.
That was what Stifyn had in mind when, as an unemployed actor living in London 10 years ago he rang up 40 of his mates from Wales, splashed out on £70 to hire a room in the Groucho Club in Soho and invited them to join SWS.
"When we started I knew a lot of people who didn't like Wales and wanted to get away to London," says Stifyn, from Rhosllanerchrugog, near Wrexham, who was in his early 30s at the time and perhaps best known for the first gay kiss in a British TV soap while acting in Brookside.
"It was the days before everyone started going on about 'Cool Cymru'. But you always feel more patriotic when you're away from home and ex-pats wanted to meet each other.
"There were plenty of Welsh societies around but I was keen to get away from the idea that meeting Welsh people meant dressing like a vicar, going in a pub with a harp playing in the corner and a lamb dinner on the menu.
"I thought it would be fun for us all to get together and have a party. I wanted to create an atmosphere which was friendly and cool and which showed it was trendy to be Welsh in London, instead of embarrassing.
"Fortunately, everyone turned up and the next time we met, 150 people came.
"Today people join because it's a way of meeting other people and having a good time - some people come looking for a flatmate, a business deal or romance. We've had two marriages from members who met through the parties over the years.
"Yes, people get drunk but there's never any trouble - the worst damage was when a piano stool got broken at the Groucho Club because too many people were sitting on it. Not that we were hymn singing at the time - that sort of behaviour is banned!"
A decade on they're still turning up - and not just in London. SWS has gone international as demand came for clubs in other cities boasting strong ex-pat communities.
First came New York in 2000, then Moscow and most recently Marbella, in Spain - "Costa del SWS" as it's known. The satellite clubs meet annually at events organised by Stifyn.
"I am a control freak," he admits. "If I didn't keep my finger on them, they'd all turn into Noson Lawens with clog-dancing and hymn singing!"
Stifyn gets invites on a weekly basis to set up SWS in other countries and reckons he could have a branch in every corner of the world now, if he had the money. Sadly for SWS, despite its success at flying the flag for Wales, funding still comes just from membership fees, supported by profits from Mr Producer, Stifyn's Cardiff-based company which organises high profile events.
Naturally, he argues that SWS is worthy of some public funding as it creates good publicity for Wales.
"The Wales Tourist Board would have paid a fortune for the publicity Tatler gave Wales, thanks to SWS," he says, pointing out the article he's framed which says, "'Made in Wales' is America's most desirable ethnic label... thanks to the efforts of Stifyn Parri, the Welsh are making their presence felt in America."
And would the brand of "Cool Cymru" in the years leading up to 2000 ever have come about without SWS raising the profile of the band of luvvies in London? Parri surely has to take some credit.
Today, ever keen to move on and keep ahead, the man with possibly the biggest - and most closely guarded - contacts book in the land, is out to woo the business world.
SWSbiz, based in Cardiff attracted 75 companies at its inaugural meeting earlier this year. Its Christmas party, at the plush St David's Hotel, Cardiff Bay, at which actor Mathew Rhys will talk about a project he's working on in Patagonia, is expected to be a sell-out.
The hedonistic thirtysomethings of a decade ago, it seems, have morphed into commercially-canny fortysomethings who see the benefits of combining partying with networking.
"SWSbiz has taken off like there's no tomorrow!" says Stifyn. "There are lots of networking clubs around in Wales but most are male orientated and if you haven't got the right badge no-one wants to talk to you.
"SWSbiz is about socialising and networking at the same time - about mixing business with pleasure, which is what I've done all my life."
Page 2: The club's big players