I agree that Seven was over-used and Kes under-used. But I also think that there would have been room for both of them in the series, it would actually have been interesting to see the inter-action between them.
As for the actresses, both did a terrific job and really gave life to the characters. I must give Jennifer Lien special credit here, considering the fact that she was more inexperienced than Ryan but still did a terrific job. Both actresses deserves best possible credits for their efforts.
I agree with this statement. If Kes had stayed on, it would have made more interesting stories, especially if she had developed a relationship with Seven; it's always fun to see two characters interact who are very different from each other.