OK, honesty is the best way to approach this so here goes.
The Banners,
I liked the Red and Black thing you have going on and they tell a nice story BUT personally, I find them, rather busy. There are too many different styles for me which leads me to look at them and get bored, lose interest.
The Wallpapers.
The first one, the Spike/Buffy one i really like.
I don't think I have a negative reaction to that one. The colouring is cool, its soft and pleasing while the choice of text/font/texture all add to a nice choice of pictures.
The second Spike/Buffy i like.
Not as much as the first one but its ok. I will say with everything that is going on, there is a rather large part of it that is blank and its position really puts me off. I also don't like the blackness, its just too abrasive against the pictures/text.
The James Masters My Way one is my favourite.
I really don't have a bad thing to say about that....I love it🙂
The Angel/Buffy/Xander one, I must admit i don't like but because of picture choice.
Yes its relevant to what you are saying, its just they all look so young and different that its kinda odd to look at these characters like that.
Otherwise its done well.
The Willow one, again, I feel its just too busy. Too many different pictures in such a small space.
I know people will disagree with me, but thats my thoughts on your work.
Some Good, some not(in my opinion remember)🙂