If there's a character I didn't like in DS9 it was Dax. Either version. She was like TNG's Deanna Troi and annoyed me no end.
I think Jadzia Dax was a failed character, but in an interesting way. As I see it, what happened was the writers had a very strong portrait in mind of who the prior host (Curzon) had been, and little direction for Jadzia herself. So progressively Curzon took over and eventually permeated Jadzia's personality to the point where you end up in the later seasons with a female version of him.
Basically this starts with Blood Oath, where Jadzia decides to fulfill a pact in Curzon's name, but gradually most of the qualities associated with Curzon are absorbed by Jadzia: a certain mischievous quality, impulsiveness, a love of partying, drinking and sex, and of course the affinity for Klingons. Sisko's affectionate "Old Man" (a reference to his friendship with an aging Curzon) is increasingly appropriate as the show progresses.
What's interesting is that this would be exactly the danger if you ever had a symbiotic species of this kind: the prior host would tend to dominate the new, younger and impressionable host, if the prior host's personality had been a particularly powerful one.
I don't think this was intended, but I thought it was interesting.