Even more critical would be to include a keybed that doesnt develop mechanical noises as the ones Yamaha has equiped the SX700/900. Its a very nice and comfortable keybed, which is very easy to play near the top of the keys - great feature -, but has a big drawback: develops mechanical noises - for many like me, since when its still new.
As for the buttons I really like the fact they are made from hard rubber (very pleasing to press) but I agree this material atracts much dust that is in the air. I cover it with a lycra elastic sleeve when not playing.