Here is an opposite opinion. I also have a Yamaha PSR550 and demoed the Casio WK3500 and I was not tempted to buy the WK3500. I was not that impressed with the voices and found the styles to be too busy. The exception were the organs, which were excellent with Leslie sim, 76 keys full sized keys, also good key feel. I plan on checking out the new WK3700 and Korg PA50.
Starkeeper