Yuck!

Looks like a clunky non-Windows app pretending to live in harmony with Windows and failing (ignoring Windows themes, not using standard Windows dialogs which in turn makes it slower, etc), and then worse Apple have brought their own font rendering and it look hideous!

Why not use the built in Windows font system and rely on each user's own settings for ClearType etc, and then the fonts will look the same as the rest of the system.

Whilst I could excuse them for trying to force anti-aliased fonts on us (which at first glance often looks bad), they've done it really badly. ClearType works, this doesn't.

Other than that, I've already found a few sites that don't work. Some dynamic HTML / AJAX stuff doesn't work. Some things are sluggish.

Oh and millions may download iTunes but it doesn't change the fact it's a terribly designed piece of software. Most people only use it because they have to with an iPod (if they haven't worked out alternate ways).