The latest OS has made me want a Mac. I went into an Apple store a couple times and test drove a few of their machines, loaded with the fresh new Mac OS 10.5. I gotta say, this is probably what Vista should have been, as far as UI goes. It has a few of its own quirks that I noticed right off the bat, but not any more than Vista has. The main thing I notice is how much smoother it is than Vista (i.e. multitasking, and still getting the same UI feedback like moving your mouse over something and getting a tooltip, or clicking something and it starts showing the "loading" feedback instantly)
And accessible. Holding down Control and scrolling was my favorite part - universal zoom, it zooms in and centers on your cursor.
The sad part is the price tag - easily the most expensive OS on the planet for PC users, since it comes with a Mac.