Recently bought a SX 600 arranger and here are a few comments:
I also have the Yamaha PSR 770 and this new model is very similar in layout and operations.
The SX 600 has a better, cleaner sound with respect to the styles.
The SX 600 provides most of the voices and styles found on the 770.
The unison feature is still a mystery to me. Not sure the best way to utilize this feature.
The accent feature is definitely a nice touch ( pun intended )
My biggest disappointment is not having the ability to make draw bar adjustments to an organ voice.
Overal, for the price, this is a great arranger and a slight but definite upgrade to the 770 arranger.