The worst thing wasn't how much changed. It's how much didn't. They wrote Jadzia being replaced into the Trill mythos, but despite Ezri being a different person, she was doing a lot of the same stuff. Even manning the Defiant's console and running around with a rifle. And I found the Ezri/Bashir pairing to be painfully dull; when I saw them kissing in the turbolift, my thought was "good, they got it over with". I did not and do not have any feelings for the couple.
The ONLY good thing about Ezri as a character was the fact that she never went through any of the careful evaluation and training that precedes joining. As a result, she was trying to reconcile herself with having eight other personalities in her head, one of them psychotic. But because there's a war on and there's no time, that all conveniently goes away by the end.
They should have given Terry Farrell what she wanted, kept Jadzia, and used the idea for Ezri elsewhere. But no, Rick Berman took a quick break from running Voyager into the ground to come here and force one of DS9's few bad decisions.