If I remember correctly, I always thought that Agnes was one witch too many. Granny works extremely well on her own (well, not throughout the story, but as a character...), Granny and Nanny together work perfectly well (I found the scenes with Nanny Ogg as the only witch in it never quite as thrilling as the solo pieces Granny had within a story, though - it was a lot comical relief, which is a fine thing, but still), and with someone else at their side (first Magrat and then Agnes) whom they can excercise their strengths at, they are great.

But while Magrat still had a charm of her own, and was allowed to surprise the reader from now and then, Agnes as a character never quite rised above the initial idea of the clumsy newbit with a split personality (not excercised very consequently, too). I think I remember reading one times too many that when she turned around, parts of her needed some extra time to arrive on the new spot (or something along those line, I'm far away from home right now and can't check.) She simply had the function -at least at times- to give -mainly- Nanny Ogg someone to talk to. And for a pure background character, something she never quite stopped being, she got way too much space.