Squeak,
I owned PSR550 for about two weeks. Its sounds are great in my opinion. The registration memory operation is a bit clunky but it's still usable in live situation.

However, the internal speaker is much better on the WK1630/1800.

I also heard that the CTK671 from Casio is a lot cheaper for around $199 and I heard it has the same quality of sounds. I have to check it out again.