it's going to be hard to judge the build quality from those small pics - I personally think the 3000/1500 are starting to looked slightly dated, and the design of the S Series brings it bang up-to-date.
I admit the buttons look rather small, but I think that is just a compromise that is needed to provide a better sound system and sleeker design. If you look at the S500, it is very compact compared to the 3000/1500.
My only real gripe so far is that the display looks identical to the 3000 - I was hoping for a better (larger) quality screen along the lines of the Tyros range. So long as the diplay on the s900 can be tilted, I'll be happy to overlook this.
At the end of the day, the sound quality will be the most important factor for me, and just so long as the build quality isn't dreadful and the specs add up, then I'm getting one of these on launch day in the UK - which will hopefully be soon
Regards
James