Frater AIT over on his blog just mentioned that every Senior Adept that he personally knows is a Freemason. And he asks if he is missing anything by not being one himself. Well, it depends on what you are hoping to get out of Freemasonry. It also depends a lot on the culture of the particular esoteric Order that you belong to.
(For the purposes of this post, I am defining Exoteric Order as a group that is organized for purposes that are mundane: insurance, networking, social pleasure, etc.---daily "real world" concerns. I am defining Esoteric Orders as those organized for magical and mystical purposes---or "mumbo-jumbo and a lot of crap" as one Freemason called it [which just goes to show that not all Freemasons are going to welcome members of Golden Dawn or Wicca or OTO into their lodges].)
I know one Esoteric Order (a Golden Dawn Order in fact) where every single Adept Minor is a Master Mason. Why? Because it is in their bylaws. In that particular Order, you are required to be a Master Mason in order to become an Adept Minor. The reason is that the leaders of this Order believe that it shows a certain level of commitment to become a Master Mason. (Hmmm, I guess the lodges in their neighborhood are harder to get into than some of the local ones.)
This bylaw has an odd effect...there are no female Adept Minors in that Order. And there will never will be unless they change the rule. For some of us, this is viewed as a violation of the very landmarks of Golden Dawn which historically is open to both men and women, as well as RR et AC (the very Adept Minor oath states that one will consider both sexes equally for admission). For bloody sakes, Moina Mathers was an Adept Major (6=5), and Florence Farr was an Adept Minor (5=6) THAM---you can't tell me that they were Master Masons. This bylaw is one of the reasons that I will never join that Order.
I know another Esoteric Order where they consider Master Mason to be the equivalent of the Adept Minor Grade. I know of a member who conned his way into Master Mason (he had no intention of actually working Freemasonry after he got advanced), just so that he could jump over several Outer Order Grades and become a Hierophant. Not only did he abuse Freemasonry, he promptly left the esoteric Order he was in and started his own (promptly closing the loophole so no one else could do the same).
I myself have been a member of several Orders---outside of learning lodgekit and making some social contacts, I learned nothing of use to me for my Golden Dawn work.
So how did the idea that a Freemason degree is somehow as good (or perhaps better) as a Golden Dawn Grade arise? Simple, we can blame it on the fact that all three of the Founders of the Hermetic Golden Dawn (Isis-Urania #3) were members of the SRIA. In addition to that, the Sprengel letters state that there is an Order in Europe (though the letters also state that initiations are not done in lodge, so this part of the theory is on shaky ground). Those who hunt for Third Order typically look for another Order (the safest place for a Third Order member to hide from members of Golden Dawn is actually inside Golden Dawn itself).
The logic goes: If the Founders of the Order were Freemasons and SRIA, then there must have been a reason for it; therefore, we must give those members with Freemason backgrounds more authority.
Here is the counterargument: Membership in Freemasonry was just normal for the times. There is no esoteric significance in it---it is like having health insurance today.
In the United States, over six million men out of 21 million were members of one Order or another in 1899. There were over three hundred fraternal Orders in the United States, and about 1000 different degrees. Each year 200,000 thousand more men joined. If one includes minorities and women, forty percent of the adult population belonged to at least one fraternal Order.
The sheer numbers alone undermine the argument that membership in another Order was what set the Founders of Golden Dawn apart from the general population.
My grandmother was a member of the Woodmen of America. The neighbors on both sides of the house I lived in while attending high school were members of exoteric Orders. At least one of my teachers was a member of an Order. You cannot tell me that exoteric membership prepares one for magical work. It is only in my generation that exoteric Order membership is rare.
The whole idea that membership in an exoteric Order, especially Freemasonry, really needs to be laid to rest. If you are going to join Freemasonry, do both Golden Dawn and Freemasonry a favor and do it because you actually want to work the Freemason degrees and not because you want to boost your status in Golden Dawn.