The Casio CGP-700 looks and sounds great in the videos I've seen. I haven't seen or heard it in person yet, but I'd love to try it out. I haven't done a feature comparison between it and the Yamaha DGX-640/650 yet, but the CGP-700 has twice the polyphony (128 notes) as the DGX-640 (64 notes), or the same as the DGX-650 (128 notes).
One difference is that the CGP-700 doesn't respond to MIDI Attack Time, Release Time, Cutoff Frequency, and Filter Resonance CC (Control Change) messages, whereas the DGX-640/650 does. However, that might not matter to you, since the DGX-640/650 doesn't have any panel controls or functions for modifying the attack/release and cutoff/resonance settings of its voices, hence the only way you can benefit from those abilities is to change the settings by external means (e.g., from a DAW)-- although the CC messages can also be used in styles and song files.
On the other hand, the CGP-700 has a full-color LCD touch screen!
