I haven't seen Bill Wallace in fifteen years, so take this with a grain of salt.
I guess it depends on what he's teaching in the seminar. There's sure to be kicking and stretching, and in these he is unparalleled. But Billy boy is more than a kicker, he has a Masters degree in Kinesiology and knows more about the movement of the human body than any martial artist you are likely to meet. For advanced kickers he can tweak your technique to make it so much better it can startle you. He knows more about stretching, especially as it pertains to martial arts, than anyone out there. Any questions you might have on kicking and stretching - you just can't find a more reliable source. His opinions and techniques are battle tested, (also medically/physically sound due to his education) his career speaks for itself. He also has great punching, and knows more about relating punching and kicking together than anyone I've seen yet.
Yet, he is somewhat of an enigma. He has an odd sense of humor. It puts some people off. Kind of like the class clown from junior high that never quite grew up. A split second later he can debate just about anybody on just about anything. Sometimes part of both of those things runs through his seminars.
As a martial artist and fighter he is on a different plane than us. The people he fought against, hung out and trained with, are a who's who of legends from across the globe. I never saw Bill Wallace talk down to anybody, but he sometimes doesn't realize we don't know what he knows about the Arts. He never speaks theoretically about fighting, if it won't work on a regular basis, Bill will have none of it. (be that good or bad, that's the way it is) I first trained with him in the seventies, spent a month with him overseas on a fighting tour, and sat ringside for a lot of his fights. He was one of the smartest fighters I've ever seen. And there is nobody who ever had the skill set of kicking like Bill Wallace. Not even close.
I figure if you have the chance to train with him, what the heck, not much of a downside.