http://web.actorsequity.org/faqpublic/browse_public.asp?locator=7This is Equity's FAQ sheet on membership. It answers most basic questions.
Is being in Equity helpful? Yes, IF you have a resume and are near Equity houses that will get you hired on Equity contracts. AEA doesn't have any job placement services.
Major points of AEA helpfulness:
1) Defined job duties: you won't be taking out the trash or babysitting children or animals
2) Defined work hours: you won't be rehearsing more hours a week than you can afford to do
3) Contract Pay minimums: for a given level of theatre, you're getting a reasonable pay for your work, because the union has negotiated it on your behalf.
4) Health Benefits: if you can get twenty weeks of work in a year, you have good coverage that YOU DON'T HAVE TO PAY FOR.
5) Housing: if you work out of town, the AEA contract controls your living conditions that the theatre has to put you up in.
I'm sure others, like VSM, could add to this list quickly, but it's just what comes off the top of my head.
Major points of AEA drawbacks:
1) You can't stage manage non-union theatre anymore, to build up your resume or help out your friends. (but even for this, a producer can work on getting a guest-artist agreement with AEA) If you're union and only have one union show on your resume, and not much networking or non-pro show experience, it can be really difficult to get your second union show.
The cost:
You have to pay semi-annual dues ($59)
2% of every paycheck goes to AEA as working dues
There's an inital membership fee to join - currently $1100, and at least $400 down to open your membership.