I like Kennedy a lot! And Tara is my favorite character so it is possible to like both. Sure Kennedy is a huge brat, she can be annoying and has a bit of a superiority complex, but she's not boring! She's the first character on Buffy that is like that and she portrays that rich girl mentality very well! I actually have a lot of friends that are like her, want/get type mentality.
She also didn't exactly push Willow to love her, it wasn't even ever really said that they loved each other (which I don't really think they did). It was more of a rebound for Willow (which she needed sooner or later, in that case it was sooner) and for Kennedy, well Willow was likeable and the sex was probably great. Their relationship was convenient and comfortable at the time and I think it was a good idea for them to date, even if it was just to take their minds off of the upcoming end of the world.
It is no surprise that in season 8 they broke up, but I'm really glad they kept Kennedy, I think she is a great asset to the team, even if it's only for her authority, I like that she challenges Buffy
Plus, well, Kennedy is hot lol