It's not a good thing when the series that you're into suddenly changes its artistic direction on the covers. It can mess up your bookshelf. I hate it, hate it.
Take for example Yancey's YA series The Monstrumologist. When the first book came out, the cover was a bit vague, showing a beaker with something misshapen inside it. I liked it. It connoted the unknown and the mysterious.
Of course, the publishers had to change it. Now it's imperative to have that Printz seal (though well-earned, I believe) and come up with something cartoon-ish. Also, this same aesthetic is now employed in books 2 and 3. As far as I know, the original design has been totally abandoned.