I love it, mainly because it has the two things I really like.

First off, I can quickly make playlists that work well together, thanks to Genius.

Second, I can use a keyboard shortcut to get to the search bar, to quickly find music.

iTunes has the fastest search of any music player I've ever used. I love it for that alone. And what else should a music player do? I've used Amarok. It has too many features. It's a beast. iTunes is a background application, absolutely, but it's easy to use. And if you really need it, they've got the best visualizer I've ever seen.

iTunes was awesome when it came out for OS9, and Apple hasn't done much of anything to improve it since then. It's essentially the same application, but with many more annoying features meant to upsell.