Apple's iTunes 9 has a number of new features, such as iTunes LP (which tries to bring full-blown album art, liner notes, and so on to the digital world), Home Sharing (which allows you to sync your music across computers in your household), and the redesigned iTunes Store. Though these new features make iTunes a stronger media-playback app, it's hardly a major revamp.
With the addition of Home Sharing, iTunes finally gives you a way to sync music automatically among the computers in your home. Once you enable Home Sharing on your PC, you'll be able to access the iTunes library on any other Mac or Windows PC on your network with Home Sharing enabled. From there, you can play music from other computers and import music from those computers as well, so you can have the same music on every machine in your house.
You'll have to ensure that all your computers are on the same network and activated with the same iTunes account. This means that you can't share music between your iTunes account and your significant other's, for example, and you can't share music between your home PC and your work PC. Still, Home Sharing is a welcome addition, and it beats manually copying music files onto each of your computers.
Apple has completely redesigned the iTunes Store, and it's a big improvement. The new look is cleaner and more attractive--and easier to navigate, too, thanks to the addition of a navigation bar across the top that gives you easy access to the various parts of the iTunes Store. Album pages have been redesigned as well. Gone is the split-pane view of old (album details and user reviews on top, songs below)--instead, the songs show in-line on the page, in a more prominent location. As you mouse over songs in a list, a preview button appears; click it, and you get a 30-second preview of that song.
The rest of the interface has undergone only minor changes. While iTunes 9 is improved overall, it takes a step back in some areas. The toolbar that lets you change sort options on the fly in grid view is no longer the default; you have to turn it on via the View menu. And various controls, such as the buttons for the drop-down genre menus in the iTunes Store, appear only when you mouse over them, making them less obvious than they should be.
The new Genius Mixes are like Genius-powered radio stations: A Mix pulls together any and all songs of a similar nature in your iTunes library and plays them in random order. To group similar artists and songs together, Genius also collects song information from your iTunes library and compares it with what other iTunes users favor.
With iTunes 9 and iPhone OS 3.1, you get more control over how your apps are organized on your iPhone or iPod Touch. iTunes already let you pick and choose which apps to sync with your iPhone, but iTunes 9 makes doing so more user-friendly and more discoverable with its newly reorganized Applications tab. And iTunes now allows you to rearrange how your apps appear on your phone.
I can't think of a reason not to upgrade to iTunes 9. ITunes is due for an overhaul, though. The sidebar, for example, is starting to get very crowded, as Apple tacks more features on. Maybe in version 10 we'll see Apple simplify and streamline iTunes.
Note: This link takes you to the vendor's site, where you can download the latest version of the software.