I'm using WinAmp2.64 w/ WowThing! and WMP7 and I like Winamp a lot more. Of course, this is mostly a function of WowThing (a back-end DSP which boosts the bass and the reverb a little) and my own customized Eq settings, I suspect.
Besides the fact that the sound is better the way that I have it set up in WinAmp, WinAmp uses vastly less CPU on my computer. WinAmp is actually below the 1% threshold on Win2k's system monitor on my system, and with WowThing enabled it lurches up to a stratospheric 2%. With WMP7 it sits between 4% and 8%. I'll take WinAmp.
If you don't use WowThing, I highly recommend it. Everything about it is great except the registration process (which never worked for me), so I have to put up with a Nagware intro when I start it up. But the sound is way better than WinAmp without it.