I still use Winamp 2.09 (last version before AOL bought Nullsoft) on my main machine. I might have switched to 5.xx but I got hung up on, of all things, the thick bar frequency level display on the full mode view. (y'know, under the time) In Winamp 2 you could set it so each frequency took up 1 pixel in width (Fire Equalizer mode) and thusly it displayed a lot more information. By comparison Winamp 5 looks like "Playskool does visualizations". That, and I can't figure out how to turn off crossfades. (probably something obvious) Not that it really matters; most of the time I have Winamp locked to the taskbar at minimal size.
On my laptop I do use 5 though; it was the only Winamp version that responded when I pressed the built-in FF/Rew/Play/Pause/Stop buttons on the keyboard.
P.S. If someone knows of a visualization plugin that properly displays the Fire Equalizer mode (from Winamp 2) in Winamp 5 please send me the link.