Winamp worked. It played all your MP3s without any of the other fluff. It played your music in a very lightweight program. What was also nice was this was before every program auto-updated; so you'd manually have to go update it; except, the newer versions were adding features, not fixing bugs. If you thought Winamp was fine, there was never a need to upgrade. I remember never upgrading Winamp until way into my college years.
People have mentioned it, but I didn't change to foobar because I always used Winamp. Even when they made the modern UI, you could (and can) still go Classic. Computer space and performance weren't issues because the extra bells and whistles are easy to never use.
Only a few weeks ago did I make the move from Winamp to foobar; and it was only to see the difference. Initial thoughts are I don't like how it displays my music; but I do like the shuffle since its playing songs I never hear.