I had the opportunity to sample Mac OS X 10.5.1 (and 10.5.2) recently. Here's some random observations and conclusions. YMMV.

• Apple is obsessed with UI and it shows (in a good way). The 'fit and finish' is very nice.
• I don't like its limiting of window resize handles to the lower-right corner. I don't want to move the mouse that far, when it is near the top-left corner of a window. NADBBA. (Not A Deal-Breaker, But Annoying.)