My mum's cousin is one. He doesn't talk about it so I don't know much; but I know the basics that women were banned (maybe women-only lodges now) and that you have to be invited to join by a member; I believe it only takes one member "black-balling" you for you not to be admitted. You have to have an appropriate job and appropriate standing in the community to be accepted.
(My Dad was a bit pissed off that he was never invited to join - not sure he actually wanted to be one, but he would have liked the opportunity to turn them down!)
I don't like the Old Boys' Club network/protection thing they appear to have going, sounds very dodgy.
I have been inside one Freemason's club though - a Christmas dinner in London, I believe it was the Worshipful Company of Candlemakers or somesuch, and we were invited to be "the entertainment" (singing, nothing bad! It was Ladies Night as well) and the building was awesome. And hidden behind a facade, you couldn't see it from the street.