Well, I did buy one in the end and I am very happy with it.
Of course, there are things that Roland could have done better, such as the hardwired envelopes (one filter ADSR, one amp ADSR and a basic pitch AD for each partial), the slow scrolling through patches and the weird three-part registrations. You can work around the latter by selecting the 'Manual' registration and working with your Live Sets in the upper part so that's not really a problem. But then, it's a performance synth, not a workstation and, as such, it's definitely made me want to gig again.
I really do like the filters and whilst the JP50 is not intended to be an updated JP8, the LPF2 filter definitely gives patches a JP8-like sound. I'm not sure the SCI-like filter is very Prophet-like but it still sounds good nevertheless. The envelopes are quite fast - faster than on any digital synth I have ever owned and I've had a few - and the quality of the effects is quite stunning. It's a shame that there isn't an extra envelope that could be routed to various parameters (think Korg Wavestation, for example) because I'd quite like to assign one to Pulse Width every now and then.
I was quite impressed with the acoustic and electric piano sounds but it'll take me a while to get used to playing them from a synth keyboard as opposed to my old Yamaha digital piano's weighted 88-key keyboard. If I had more space in the home studio, I'd probably invest in a CME UF8 or something similar. Other acoustic sounds range from usable to excellent.
The area in which this synth really shines imho is the synth presets, which are very good and cover almost every genre since the 60s, from cheesy Moog solos to more recent trance or whatever, through 80s synth pop, etc. There are so many to choose from that you are bound to find something close to what you are looking for.
Programming-wise, the main parameters are quite easily accessible and can be edited quickly and efficiently for the most part. For deeper programming, you have to rely on an external iPad or by scrolling through a vertical list of logically arranged parameters. Because I have no intention of buying an iPad, I have created my own Cubase device map and it works fine (see my other thread).
To cut a long story short, I can't get my 15-year-old daughter away from the Jupiter 50, on which she uses the acoustic sounds and I have already spent hours of fun programming new patches using the on-board synth engine. This thing has a lot more potential than meets the eye, particularly if your experience of it is limited to an hour or so in your local music store.