The monsters, there's no doubt. As a 99% DM, the new art has really jump-started my descriptions and ideas about how the beasties act. As comparison, I like when WotC does the Alumni articles and puts old art next to their new versions, really shows how far the game's ascetics have come.
As for Mialee, I like her. A little 'off'? Here and there I suppose, but I felt it was to envoke the fact she wasn't just a human in elf-ear makeup. And different artists do her different levels of justice. This pic is really cool IMO!
-DM Jeff