Having just tried the beta, I have to say the new GUI looks pretty awful, especially when run on a Windows 7 computer. It looks completely out of place. I am at a bit of a loss as to why Piriform decided to change the UI, it's lasted this long because there's was nothing wrong with it in the first place?
TheAsian is right, flat UI's appear to be quite unpopular generally and I can't quite understand why developers try to flatten their software interfaces when the look is so disliked. It belongs on Windows 8 maybe, but certainly not Windows 7.