First reactions: it's nice, but I'm not an addict just yet. Based on my experience so far, I'd rank it above Microsoft's player, but below WinAmp (my current player).
As an actual playback engine, it seems fine: I've only gotten it to stutter once while writing this, browsing the net and ripping a CD in the background. It handles song-to-song continuity reasonably well once the default fading behaviour is switched off (though not as well as WinAmp: the transition from "Sweet Thing (Reprise)" to "Rebel Rebel" still has a gap). The search functions seem nice, though I expect I'll appreciate them more once I have some actual tagged MP3 or AAC files in the playlist (right now, it's just playing Bowie tracks in WAV format, which format doesn't support any ID3 tags).
As with QuickTime—which I still think deserves its place in the Interface Hall of Shame—many of my complaints have to do with Apple's unwillingness to abide by the interface conventions of the window manager. There's a system menu, but you can't get directly at it with a mouse (you have to right-click the taskbar entry, or else use Alt-Space). The button at the upper right that should maximise the window instead reduces it to "compact" size. The window can only be resized from one "magic" corner (should be able to do this from any edge), and in any event the window isn't resizeable below a certain minimum. Does the MacOS version break the rules of the Mac's window-manager this badly?
(In fairness, Microsoft's dreadfully irritating Windows Media Player 9 also won't resize below a certain minimum. And at least iTunes actually honours my preferences as to which sound card I want to use for output, unlike WMP 9, which completely ignores this Control Panel setting.)
iTunes does have some things in common with WMP 9, though: both want to take up way more space on my desktop than they really need to (compare to WinAmp, which is a model of economy) and both seem to want to take over as my primary audio-playback engine (I unchecked the "Use iTunes as the default player for audio files" option in the installer, but it was still checked when I got in to iTunes' preferences). Apple still hasn't gotten over its "let's put the app into the Quick Launch bar without asking the user!" problem: after very nicely asking whether I wanted desktop shortcuts, the installer dropped QuickTime and iTunes into the QL bar without so much as a by-your-leave. (It also sets up two autorun programs: how about letting me decide whether I want to run iTunes or QuickTime at system startup?)
Any more gripes? Lessee...
• I wish the "Preferences" had point-and-click help: it's annoying to have to cancel out of setting preferences to see what each setting does.
• I'd like to be able to see at least some of my playlist in the super-compact view.
• It's slightly frustrating that I can't figure out how to sort tracks in an arbitrary order (like, say, the order in which I added them to the playlist).
• There doesn't appear to be any user input into the Visualizer other than "on" or "off": if there's a way to, say, pick a specific display pattern, I haven't found it.
That said, I'll keep playing with it, and try to figure out which of these are just new-user gripes, which are justified, and which, if any, are fatal (i.e., I-won't-use-this-app-because...). Feel free to post comments about iTunes' good points: I'm sure I haven't found them all yet.