I played at a WK 3500 in a store (local reps) last week, for about 30 minutes. The sounds are WAY better than my 10 year old casio, and it was something I expected, however, they sounded more comparable to PSR 1000 for me, nowhere near the PSR 1500 and up.
On the other hand, the styles were not so good, at least compared to the one I have (CTK-750, Casio flagship in 1994, same sound engine as early WK models, 128 styles, GM tones and another 50-60 interesting sounding "magic tones"). Some styles were exacty the same. The guys at Casio should have worked more, and it is a pity they didn't include 4 variations.
The keys had a different feeling, but I think I liked it, and there are plenty of them, which is a nice thing.
I agree that it is a step up, but IMHO it is better suited to someone who has never touched real keyboards before (eg music student etc). Anyone with some experience will probably outgrow it easily. (excludind master keyboard use.)
For me, it wasn't a dissapointment, but not a significant step forward either. I play at home, for my own recreation, and I probably expected to hear something more "fresh".

The price: MORE THAN 800 EUROS incl VAT, in Athens, Greece. I can mail order from the UK to my doorstep, for less than 500 Euros. The local Casio representatives, should pull their finger out. Price for MZ2000: 1670 Euros, didn't get to hear it, they had no adapter available at the moment.