You pay an annual membership of less than $1/day (and even less for students). In return, you can take as many sailing lessons as you want, and sail as much as you want. The caveat? The only boats you have access to are Mercuries. The Mercury is a great beginner boat (much better than the Tech Dingies at MIT Sailing across the river), but it is kind of like a U-Haul on water. It's really not a good boat for anything more advanced.
Once you've outgrown the Mercury, there's really nowhere to go. CB has more advanced boats, but to use them, you have to pass the Jib exam. The Jib is a pretty easy exam (man overboard drill, basic jibes and tacks), but to take it, you need to wait for very special weather. There needs to be enough wind that the Charles starts to have whitecaps, but not so much wind that they call all the boats in. Many people wait a year or more before the right weather comes up.
If you come in with prior sailing experience (as I did), and you have a job, so can't go there on the one day each year you get jib test weather (as I can't), you're basic SOL. It doesn't matter if you can win boat races, pass any of their exams without a rudder, or teach a class on sailing theory. You're basically stuck.
I have a one-year membership, which I think was very worthwhile (I learned to windsurf, and I did experience a new boat -- the Mercury), but I don't plan to renew. I'd recommend it to beginners learning to sail, but not to anyone more advanced.
Pros: Good for beginners, nice lessons, inexpensive
Cons: Doesn't scale to more advanced saling