I had to buy a stage piano for our church a year or so ago. I spent a day trying out the various offerings and bought a Yamaha P200. The Rolands were ok but the Korgs were uniformly nasty and thin (brittle sounding). This was a shame because the Korg pianos generally had better facilities.
I haven't regretted the decision since.
On other keyboards (that weren't specialised pianos), I always prefer the Roland sound - this may be partly due to the improved keyboard action which is always an important consideration when trying to play a piano but not such an issue when playing other sounds.
I also have a Kurzweil and love that piano sound too.
I have never heard a Korg piano sound I liked yet (yes I know it's a mater of personal taste). Even the piano expansion boards don't cut it for me. Again a shame as otherwise I might like a Korg (and I do own other korg keyboards without pianos like the Z1) but I use the piano sounds a lot in my performances and particularly when composing. The poor piano sounds in Korgs therefore puts me off using them and it seems to me that whilst Yamaha lead the field in specialised pianos (they make the acoustic versions too after all) they don't seem to put the same care into their non-pianos such as the PSRs and Tyros sounds.
[This message has been edited by andrewpowell (edited 11-02-2005).]