People seem to have a love/hate relationship with iTunes. Windows users were used to it being very slow and buggy. The latest version 12 seems to have put that behind it, but as a music management tool it sucks. Of course that depends on what you mean by music management.
To the average user who buys only from the iTunes store and for whom the app is just a means to manage the device with minimum fuss it is no doubt absolutely fine. It does suffer though from the Apple “we know best” approach whereby you can use it any way as long as it is Apple’s way.
For people wanting more fine grained control of their music it is not so good. Other apps offer better control of tagging. At least on Windows, there are no plugins for iTunes. The interface cannot be customised or personalised. Don’t like the player at the top? Tough. I can almost hear someone at Apple saying that these features are not necessary as Apple has designed the best user experience. However, we are not drones or isheep.
There is no integration with tagging services like MusicBrainz, Discogs or Last.fm. No support for commercial lossless formats like FLAC. No find/replace tools for editing multiple tracks. No means to copy meta data from one track to another. Little in the way of library management. No tool to find tracks with missing artwork. No tool to find tracks with dead links easily. I could go on.
There are some good points. The auto playlists are fantastic. The Album cover view is great with the click to show track listings. However, I feel the majority of apps out there like Media Monkey, Music Bee, Foobar etc are better and more customisable to personal need and choice.
So why do I use iTunes? Simple, I have an iPhone and an iPod. While I would very much like to use these with my music manager of choice, Apple does not support it officially. Media Monkey is the only one I am aware of that offers both comprehensive tools and bi-directional sync to IOS devices. However, for me, since IOS8.3 I have had no end of problems syncing with Media Monkey. I would really really like to use it with my iPhone but the two will not play nice no matter how hard I try. Even if they do, there is no guarantee that the next IOS update will not break it. In this particular battle, the plug and play symplicity of iTunes wins. I have managed to overcome the main shortcomings by writing a helper app to add find/replace, meta copy/paste functions etc.