I just sold my S90, and my Nord G2X and recently my old Kawai K3. I was thinking about retiring from music but I couldn't do it and bought a K2600XS. I still need to go and get it so I've never actually played one of these.
The S90 has a great keyboard and some great Rhodes sounds. There's a mid-70s Rhodes sound that is exact. My S90 was sitting above my 1974 Rhodes so the comparison was easy. Some of the acoustic guitars and electric guitars are good, a few useable organs and a really nice electric bass.
There were some problems 'though. Strings with sampled vibrato was one of the dumbest things I heard, and the user interface to the synthesis parameters was very old school. The much-heralded S700 piano didn't impress me at first but after some tweaking it became very useable. I have a .zip file of the tweaked piano if anyone wants to email me, I can send it. You can hear it here
http://www.cdbaby.com/cd/piscione. The gang at sninety.com seemed impressed with it.
Even 'though some of the sounds were great and the action was very nice, I would not buy another Yamaha synth, mostly because of the S90ES. Yamaha should've fixed the issues with the S90 instead of releasing a new synth that was really just a fixed S90.
Randy