I agree with most of the sentiments expressed here against joing the "union" (which it is not.) A real union has clout and effective bargaining power, and are respected (if not feared) by organizations, associations and companies whose workforce are unionized, which is why it has the clout and bargaining power to begin with, and, its members are united and back the union.
(Try getting a quorum [let alone several 100 or 1000] musicians to agree on anything!)
On the other hand the AF of M and their locals, are basically a cash grab, and are virtually useless, if your goal is to play clubs, and try and eke out a living as a professional musician. In the old days when hotels had steady engagements (4-6 consecutive nights of work) it was generally required to be "union" or both the musicians and hotels would be "blacklisted". The black list consisted of union reps telling the hotels that unless they hired union musicians, they wouldn't be able to get good live entertainment. They also published the names of the blacklisted / unfair establishments in their semi-quartely magazines, and in their newspaper called Internation Musician"...and suspended and fined their own members who violated the blacklist. They have always been notorious at policing the conduct of their own membership much more so than any questionable establishments hiring live music.
As a result of blacklisting, the clubs and hotels thumbed their noses at the union, and instead hired non union performers who were happy to have an opportunity to perform regardless of the fee. This undermined the efforts of the union and the union musicians. But to each his own.
The best thing is to do a bit of research and decide upon and target the type of venues you want to perform in, and in doing so, determine these venues policies. If most of them are union, join the union. If most of them aren't, save your money.
You only need to be union if you are requiring help in securing performance visas outside of your country, or if you are going to be doing commercial recordings (ie: jingles for radio and television, sound track work) and /or performing on television and/or concert venues.
You should also have some sort of talent or skill that is so highly specialized that no one else can do what you do, if you want to have any clout in the bargaining process. Otherwise as far as clubs, etc. are concerned there will be another "cable-car" along in five minutes.
The union and AF of M is a toothless lion, and serves only it's trustees, stewards, secretary treasurers and other executive board members.