Mark, I've heard this discussion before about low-rate MP3's cheapening the sound quality of keyboards and honestly my abused ears probably aren't the best judge anymore. The MP3 I made sounds pretty much like the ES4 to me even though it was meant to be a rough demo of secondary sounds with a small sample of my tweaked acoustic piano sound thrown in. I haven't heard a Korg SG-1 in so long I couldn't make any comparison (except that today's ES4 with 96-note polyphony, string resonance, damper pedal effects, etc. probably has a slight edge over old Korgs). I used to swear by my Kurzweil PC-88's piano sound but I recently played one and couldn't believe how crappy it sounded... times have definitely changed.

Also there are many parameters on the ES4 that can be customized and I certainly tweaked the snot out of them, so the only way to critically judge it's sound is in a quiet music store and make sure you have done a factory reset beforehand.

I love the way the ES4 sounds on stage and for practicing in my studio, by even so I use it to control a softsynth piano when recording (Colossus' Steinway D sample) because the softsynth sounds better to me and is quieter. And I'm probably going to migrate to Ivory eventually. FWIW: YMMV.