I recall a fairly heated thread on this point. David Allen's own post seemed to put things to rest when he explained the business relationship the two of them had.
I bought the Take Back Your Life book. Yes, there are many similarities, but there are enough major differences to make this book an important addition to the literature on personal productivity.
Sally use a very different approach to the calendar than David. She recommends scheduling everything on the calendar. Since you are able to drag and drop items from the task list onto the calendar, this is an easy thing to do. Personally, I prefer David's philosophy of keeping the calendar pretty open, but if you like the idea of having it all on the calendar, Sally's book is a good one.
I agree with you. While there are similar concepts to GTD in the book from their earlier collaboration, there are many other great tips for managing things with Outlook beyond GTD and was definitely a worthwhile buy.