Sandra is one of the most well-known names in unschooling. She can be perceived as VERY tough on individuals who post on her Yahoo Groups list, Always Learning. The list is intended for very pick-apart-analyze-the details-of-your-thinking discussion of the topic of unschooling. It's my opinion that Sandra comes across as tough on posters, because she is fiercely dedicated to protecting children from what she perceives to be harmful practices engaged in by well-meaning (or not necessarily so well-meaning), but unenlightened parents.
In person, Sandra is quite engaging, and not quite so tough-seeming. I've had the opportunity to see her several times at conferences, and she is very thoughtful and helpful in her speaking engagements.
I've read her list discussions and wisdom for years, and some of the stories that she's relayed are so often quoted, that I will remember them like a story that is passed down in a family from generation to generation. Her book, the Big Book of Unschooling, is a collection of those stories, arranged in topical format. It reads like a collection of short stories, rather than a guide or instructional manual to unschooling. I find it is fun to browse through occasionally, even now, when my children are getting older.
I consider Sandra to be part of a revolutionary group of thinker-practitioners when it comes to childhood education. She and a handful of very intelligent, caring, daring parents chose to experiment with their children's learning, and then to share this experience with the rest of the world. A movement has grown out of it. Words that she and her peers have written have, literally, changed my life in profound ways. I look at children, and human relationships, and motivation and contentment and the purpose of life differently as a result of these (mostly) women.
I encourage anyone who is interested in this topic to read this book, and Sandra's other book, Moving a Puddle, which is also a similar collection of stories.