They are one of a few...from what I have heard they are a bit on the more expensive side for what you will end up getting.
Mainly there are gonna be two kinds of CSA's...Meat and Plants...
I am thinking about joining a Meat one this year...
As for plants...
There are a few more options, but the one thing that matters is where you live and where they will drop off at. I look for Ann Arbor pick up and drop off. You may want to ask around your local farmers market. I know Maple Creek has a big Royal Oak presence.
Another thing to watch out for is that some CSA's will ask for you to come out to help work. I have no problem with that....but some may.