Most people describe Kennedy as Hard-Headed, A rebound for Willow & Spoilt.
As for Kennedy being Hard-Headed. What slayer isn't? I mean ever slayer has to be Hardheaded. Buffy is, Faith is and to some extent so was Kendra.
As for her being spoilt, To me she really didn't seem that spoilt Yes she did seem to be a bit...Insistent on some things but that hardly qualifies her as Spoilt.
As for her Being a Re-bound for Willow. Yes I believe that the relationship was a little rushed compared to the Tillow relationship and the Woz relationship. But here's the thing. She's not Tara, and she never will be, there will never be anyone who could replace Tara.
For me I think they did the right thing by making Kennedy the opposite of Tara because if Willow's new Girlfriend were like Tara then to me it would have been like the writers would have been trying to replace her. Which to me would have gave the feeling that the writers were being a bit disrespectful to the character of Tara.
Yes, I have had moments where I disagree with Kennedy, but I think everyone who likes a character has moments where they think what the heck was he/she thinking? But Overall I like Kennedy.