It all comes down to different ways of doing things. Some people like to manage their music more manually, copying over mp3 files to a folder. I am in this category, because I like complete control, and some of the things itunes does seems to be confusing. Also, I don't see the need of having mp3 files on a hdd, then stores on my iphone, then also stored in the itunes music folder. I don't need 3 copies of a music file. This is one part that frustrates me. Another frustration was silence between tracks, which itunes did not used to support.
However, I completely understand why most poeple like itunes, because most people aren't like this, and itunes provides a very simple interface for them to do it.
Saying that, the Windows version of itunes has not been as good as the mac version. And even on the mac, it seems to pause when copying over lots of large mp3 tracks, although this has gotten a lot better in the recent release.