I just bought two PSR-SX900's as I make a living playing these things, and I have two PSR-S970's I need to sell. I got a deal that was too good to be true from WWBW.com.
There are a lot of exciting new features. I wish I had more time to explore this thing, but I really got into playing acoustic guitar.
I love the assignable buttons. I love that you can get live control buttons to change assignment via registration.
One question I have is this. With the PSR-S970 there were two pretty easy ways to adjust the mic volume. I don't have my keyboard in front of me, but I believe on the S970 you can adjust mic volume on the default screen by pushing up and down the value buttons below the main screen. Also, there was an assignment for mic volume in the Live Control knob menu.
On the SX-900, there is nothing that appears on the default screen for mic volume. There doesn't seem to be a Live Control assignment for mic volume. It seems like the only way to adjust this parameter is through the Mic menu and then using the touch screen and the knob in the back of the keyboard. I often fine tune my mic volume, and I will miss the old way unless I just haven't learned the SX-900 yet. I just got it, and I'm daunted by all the new styles, voices, and features. WOW.