The Bad: White is too bright; grey was better. A new highlight style (like the one on iTunes DJ) would make it better. Progress thing is crappy with the fill, as is the volume slider. They just don't fit. "Sunken" search field. Title bar looks like it's running at 1024 colors or something; really unrefined and gritty. White background looks unrefined, almost like it was rushed. IMO, it looks like MS did that part. Song-name bolding either too bold or not smooth enough. Store WAY too slow (at least at initial run). Titlebar section too fiddly (like the back of the Ferrari F430).
AND PERHAPS THE BIGGEST LETDOWN SINCE SNOW LEOPARD
iTunes 9 is not 64-bit. Probably still Carbon, too.
The Good: The new icons are nice. Song name bolded. Tab-like thing at new store. Many elements of Marble/Illuminence or whatever starting to emerge. That's about it.
Overall: Looks a lot like Windows XP Media Center, i.e. glass effect overused. Still has some really rough edges, but it works.