I'm going to ignore the hardcover cover of The Looking Glass Wars - the really big image - because it just doesn't fit in with the covers of it's sequels. The paperback one fits in much better.
I, personally, enjoy the covers. They're not ZOMG amazing, but they're very well thought out. The layout never changes; the characters and color schemes are the only things to alter as you move from cover to cover. The paperback Looking Glass Wars cover is my personal favorite; I think they did a fantastic job of capturing Underland.
What say you?
