The S700 and S900 are definitely "worthy". The only thing is, it's not the successor Yamaha claims it to be. To me and a lot of users on my own forum it's totally unclear why there is a PSR3000 _and_ a S900. Both instruments overlap a great deal, and each has its own benefits over the other.

I suspect the same goes for PSR1500 versus S700. So if you have a 1500 or 3000, the general thought is: "Don't". If the S700 or S900 is a real upgrade to you: go for it.

And if you'd rather have the S900 than the S700 but can't afford the S900 right now: Just wait and save some more money. Even though you may never know exactly what you miss out on if you go for the S700, you'll still have that nagging idea of "but what if I had bought the S900..."