In the old days, most of the credit union memberships were based upon employement (although, even there, the name was frequently more limited than the organizations listed). In recent years, many have shifted to a geographical designation.
That being said, one of the larger ones in this area recently converted to a S&L.
Seems that most credit unions are pretty easy to join. The small town I moved into a year ago has 2 different ones that said I could join. I did join one of them.
I have two Visa's with a credit union. I don't get any of the perks I see advertised. But I get good interest rates and the other terms are much more favorable. Worth more than the airline miles and stuff like that.