I'm considering it this year. Through localharvest.org I found the web site of a local organic farm that does a CSA where you can actually go to their booth at the Farmer's Market each week and pick out what you want! They only put limits on the expensive or labor-intensive things (they give the examples of watermelons and green beans), other than that you can take whatever you will use.
It sounds really neat, but I want to see the quality of their stuff before I drop $500 for the season... the farmer's market starts this weekend, so I'm hoping to find them there before the CSA starts in June so I can take a look.