The improved device management is by far our favourite feature, taking the pain out of organising and removing apps.
Coupled with improved sync options and the ability to automatically stuff empty space with songs that means iTunes 9 makes it much easier to sort out your iPod or iPhone than before.
Home sharing is a big improvement over iTunes' previous approach to library sharing, and the new column browser speeds up navigation of big libraries.
Genius mixes are nice, but why can't we see what's in them and edit their contents?
We're not overly keen on the ability to tweet or update Facebook from the iTunes store, and we're not convinced iTunes LP is a particularly good idea: the iPod killed the album years ago, so it seems a bit late to try and resurrect it with some fancy pictures.
Last but not least, it seems churlish to add an application management screen but not give users the ability to delete unwanted standard applications, such as Stocks.
iTunes 9 feels snappier, the column browser is a much-needed improvement and the Home Sharing feature works very well, although on our Mac at least iTunes 9 doesn't seem any less crash-prone than its predecessor.
Nevertheless, the vastly improved device management makes this a worthwhile upgrade.