I'm somewhere in the middle when it comes to Kennedy.
I personally would have liked to see Willow single in the last season and really focus on herself and her redemption. I know this was hard due to the season-long arc going on and how she ties into it, but especially after having such a long relationship with Tara too.
In some ways, I think she actually fit with Willow due to her personality. Willow and Tara found one another during a time when Willow was going through a hard breakup and figuring herself out. Willow grew as a person, and so did her powers. I think after Dark!Willow, there is no way she could have been with someone like Tara. I liked Tara most of the time, but she was still kind of demure. Kennedy worked because she wasn't afraid of Willow, and her being determined too.
I also think after all the spats Tara and Willow had about the magic that it made sense for Willow to be with someone who wasn't a witch. Kennedy didn't understand most of it, but she was still supportive, and it only bothered her the time Willow sucked energy out of her to open that gateway.
Do I think Kennedy was the best choice for Willow? No, probably not, but I didn't really have a problem with her. I know most people find her annoying, but she spoke up a lot when other characters wouldn't. I think what I personally liked about her is that she was willing to speak up, willing to take charge. Although being kind of bratty, she had the makings of a leader, and we did see this a little at times, which is one of the reasons I think she clashed with people so much.