I used to be anti-iTunes (WinAmp 4 life!), but at some point my library became so big that it was literally impossible to keep track of it all in a nice and orderly fashion. I’m currently at over 18,000 songs (110 GB) and iTunes is the best tool out there for managing such a large library.
The smart playlists feature is really powerful for filtering data (eg: “All songs greater than 2 minutes but less that 8 minutes that have a genre of Alternative that came out in the 1990s that I have rated 4 stars or above that I have listened to less than 3 times, but don’t list anything by band X”).
I like the ease of being able to just import and play.. not having to worry about naming/tagging/filing into folders/getting artwork for everything.