While I haven't tested UI with my Mac, on my Windows machine, I've noticed a LOT of the demos simply don't work.  Like the draggable demo where a block is supposed to be contained within its parent.  I can drag it anywhere.  Plus tons of controls show the radio button when they are not supposed to.  For example, on the Themeroller, there is a radio button on the upper-left of "Font Settings".  Etc.

On the autocomplete demo, the drop down window appears in all sorts of crazy locations.  It's almost never under the text box.

I realize I am using a beta version of Chrome but is Chrome really that off?  

FF 3.5.5 seems to work a lot better.  Although, on the draggable, you can pull the child out of the parent if you do so while making the outer box scroll.

Anyway, the UI looks to be a fine product but I'm a little concerned on the Chrome issue.