I came to like this character a lot in later seasons but a lot of ppl I know, who are even more hardcore treky then me, could never get past the idea that she was a replacement for Kess and the fact that Jeri Ryan, the actress who played Seven, was dating one of the producers at the time. However Seven became a really interesting character once they began exploring her humanity and stopped just using her for a ratings boost. I also felt the actress was very talented and played Seven very well.
Kess always seemed to be a person really comfortable with herself and was a normal startrek type character. Seven had depth in that she wasn't perfect and had good reason for it. She always struggled between two very different worlds. She also lasted to the last season when the writers actually started writing better.