I just did two privates with Shawn on Monday and Tuesday of this week, while I was out in Los Angeles for work-related stuff. He is a truly amazing teacher, and literally EVERYTHING that he showed me has worked yesterday and tonight in class. He has a way of converting problems that I think are huge into fairly simple solutions that I can't believe I didn't think of on my own.
You don't have to be flexible to use the Williams Guard. At first I thought you do, but you just have to escape your hip a little to the side you want to work on, and that will allow you to get your leg up high enough without being very flexible.
I picked up the DVD while I was out there, and it's excellent. Anyone who can get the DVD - and train with Shawn at some point - should do it. Once you train with him once, you'll make it a point to go back again.