I love the covers for the His Fair Assassin Series. They have tough, kickbutt girls on the cover who just look awesome! I love this hardcover for Dark Triumph by Robin LaFevers:
But I'm not so sold on the paperback cover. I still like it, but the girl just looks less tough to me.
Midwinterblood by Marcus Sedgwick
This is a cover change I really like. The hardcover is OK, but it looks a bit weird and bland to me where I think the paperback has a more romantic and mysterious feel to it.
A Soldier's Secret by Marissa Moss
Nonfiction doesn't always get a cover change, but I love this one! The new cover tells me there's going to be intrigue and an interesting story inside and I just find it more appealing.
Sun and Moon, Ice and Snow by Jessica Day George
This is a fairy tale retelling I read a few years ago. While I like the original cover, I think the new cover matches the rest of the author's fairy tale retelling covers and also looks more historical and fairy tale-esque than the original. I'm a fan of the cover change.
Any recent cover changes you like or dislike?