They do work, and I can speak from experience here. There are a lot of different types of jelq's, and they're generally used mainly to gain girth more than length. Length, from my personal experience, is easiest gained by hanging.
A good site I have found to use for this sort of thing is
www.thundersplace.com.
Personally, I have gained 3/4 an inch in lenth and 1/3 an inch in girth doing these exercises very seldomly, mainly to just experiment with.
So my answer is, yes, they do work. Good luck!
Scott