Since a few days I own the PSR-SX600. I am happy with the sounds and styles so far. Also the keybed and the general workmanship gives no reason for criticism. The 100 MB Expansion Memory is a bit limited, but sufficient for my needs.
I know that I would have got the SX700 for "only" 300 EUR more. But I mainly choose the SX600 over the SX700 because I don't like resistive touchscreens and I prefer to work with real buttons. However, my main reason to choose the SX600 is the USB audio interface, which integrates seamless with my iPad. Since I like to use the keyboard together with synthesizer apps like Drambo, this feature is very important to me. Using only one cable for audio and MIDI connections is simply great.
Unfortunately, the SX600 seems to be very picky when it comes to USB sticks. Only a few of my 8 USB sticks are recognized, with 3 older ones I sometimes (!) had success. Of course I know that the USB sticks should be formatted on the keyboard. But if they are not recognized at all, I can't do that. I also reformatted the sticks on the computer, even without success. A brand new Verbatim thumb drive that I bought yesterday is not recognized either
BTW, are there any other SX600 owners here in the forum?