I'm no fan of the PlaySkool Tile World look, but forget the aesthetics for the moment. The fact is that with the new GUI, it's very hard to tell what you're looking at. Things are pale, and tend to vanish. It's difficult to determine where the focus is - focus on the tabs, for example, seems to be the reverse of what the colors would indicate. In short, the new GUI is less usable.
I'm not going to threaten to take my marbles and go home - CCleaner is still a tool that every user should have. But this redesign was misguided, and unnecessary. A step down.