A friend of mine went the other weekend. Here's what he said:
"HNY was good. It is a very rigorous practice. My legs were really tired after the class. As usual I joined the group going to dinner after the class, which was very social."
I do have to say that $25/session for HNY is pretty damn expensive.
If you're looking for a good beginner yoga class, I would recommend taking the Fundamental classes with Glenn at Sahanna (http://www.sadhanastudios.com/
Worcester Street/Washington). This is how I started out. Glenn is a great teacher. Classes are $15 or you can get a 10 class card and save more.
Actually they're having a beginning workshop (Fundamental One) tomorrow (Sat) if you have the time:http://www.sadhanastudios.com/workshops.htm
The Sahanna Fundamental classes are a good way to learn the basics. I have not been to the HNY classes yet but it sounds like you should know the basic positions before going, but maybe I wrong.
Good luck. I wish that I started yoga earlier.