It seems to me that the poseable sym option is just working with the standard and inflat brush?! With every other brush it always fails to work correctly after a few strokes overlayed, especially clay brushes, or brushes that somehow flattens the surface or do more than just pushing and pulling the geometry.
Also I´ve found that it´s impossible to work with the movetool at the centerline. It works fine with the first stroke, but the second will always break the symmetry.
I hope Pixologic will improve this feature a lot, as it is so important when you want to pose the model as early as possible and work from there.