Well, AAC is a lossy, DRM-encumbered format that's not well supported by er... anything but iTunes, but the fact that it's based on WinAmp 2 warms my heart.
Most of the cool WinAmp plugins are for v2 and were not ported to v3 (e.g. Milkdrop). WinAmp 3 was in my experience not terribly stable...