And another question: did Bach (and his contemporaries) use these symbols himself, or were keyboard players expected to use their taste and common sense to ornament music? Which would imply that most of the ornaments we see in sheet music were put there by editors rather than Bach himself? I seem to remember having seen different editions of the same work with completely different ornaments, or the same ornamentation symbols, but footnotes that explain different ways of playing them.
I am generally a bit confused about ornaments. I must confess, I don't really like them much; they seem to interfere with the music itself, especially if a piece is heavily ornamented. For an amateurish player like myself it is of course also easier to simply omit them... :-)