Unfortunately, the channel that shows Dr Who over here doesn't always show them in order, but there was one episode with "Doctor Donna". Granted, that was an aberration but I didn't care for it at all. I would also worry that the Doctor's caustic side (which they all have had) would come out as plain bitchiness in a female (writers do seem to have a hard time with that sort of thing).

I just don't see any need for a female Doctor. I haven't always been fond of the actors portraying him, but I'm quite sure I would lose all interest if he were a she...