Ian,

I bought the PX-5S the last month of May; after six months of use here are my thoughts:

Pros:
1- incredible action; I am not an expert on pianos, but according to people who have played all the digital pianos available (including Yamaha, Roland, Nord and Kawai) this Casio gives the player the best connection with the music played, to the point that you tend to forget that are playing a digital emulation of the real thing.
2- massive polyphony (256 notes) that enables you to layer many sounds without running out of notes.
3- Synth engine: incredibly warm and almost analogic in quality; go to the Download section of the Casio Music Forums and listen to the demos of some of the sounds uploaded by actual Casio users and you will understand what I mean.
4- Excellent master keyboard, with lots of possibilities and flexibility.
5- Low weight: almost incredible for a digital piano with such a nice and firm keybed: 11 kg (24 pounds)
6- Price: 999 USD.
7- Can run also on batteries.

Cons:
1- Not everybody likes the black and white look.
2- The display is small, but there is an excellent editor for Windows.

Speaking of sounda, the acoustic and electric pianos are excellent, while the guitars or other orchestral sounds are just ok. If you go to sites like Keyboard Corner right now the talk of the day is the new Yamaha CP-4 and how it compares with the CP-5; everybody has a different opinion (like happens for real acoustic pianos), but these are digital pianos that cost (and weigh) like two PX-5S.
In my case, I bought the Casio almost at the same time as my Kronos 61 and I got both for the same price of a Kronos 88; in fact I use the Casio to drive the Kronos and this combination is just magic, especially because the Casio, with its limited depth, fits perfectly under the Kronos.
So, I can recommend the PX-5S to everyone looking for a digital piano that can be also used as a complete master keyboard. This combination alone would justify the purchase; if you add the low weight, the huge polyphony and the almost unbelievable price you have a product that, as far as I know, has no rivals in today's market.