Some thoughts on this, from an enthusiast, collector, and amateur player of odd and arcane keyboard instruments.

On the subject of one string per hammer: while this is novel for a modern acoustic piano, it is not especially so for other members of the family. Smaller harpsichords, for instance, are always strung like this, and even in larger ones having multiple "choirs" of strings, the number of strings being plucked simultaneously by a single key can always be configured, typically through levers or pedals.

In the 1970s Yamaha brought out line of single-strung stage pianos (CP-70, CP-80, etc.) that were popular with rock players because they were relatively portable and significantly faster to tune. (Kawai also had an upright of similar design.) These were not loud enough to be played acoustically, but instead were fitted with electric-guitar-style pickups under each string much like the Una Corda has, producing a similar sound. In fact, as I listened to the soundtrack on the video, I was struck by the resemblance of the sound not only to the Yamaha CP-70, but to the Rhodes electric piano, which has rigid steel tines instead of strings, but also one to a key.

I like the ability to swap in various felts to modify the timbre; many modern harpsichords have this facility also, with something called a "buff stop." Player pianos in the first half of the 20th century frequently featured a lever that would lower a comb of felt with little metal rivets between the hammers and strings, producing a "honky tonk" or "tack piano" sound; presumably one could fashion something similar for the Una Corda.

The open, vertical design reminds me of the beautiful Clavicytheriums produced by the American harpsichord builder Steven Sørli (http://www.lautenwerk.com). Here's a video of one: https://www.youtube.com/playlist?list=PL89758D803857A19B

Yes, these are quiet instruments, but the late-Renaissance and Baroque music one would typically perform on them is much better served by this sparse clarity than can be produced on the modern grand piano. I suspect that the Una Corda would be similarly friendly to this repertoire.

Another benefit of the single-stringing is that, presumably, a capable player might reasonably expect to tune the instrument themselves, permitting the setting of very specific configurations other than twelve exactly equal divisions of the octave tuned to A=440. For acoustic and historical reasons, a lot of pieces really bloom in a certain way when you can do this, but on the modern grand piano it's a time-consuming task usually best left to a professional tuner/technician. (Because of the relatively unstable nature of their instruments, harpsichord players, like harpists and guitarists, have to learn to do this themselves early on in their studies, and thereby gain exposure to various temperaments and reference pitches, e.g. Werckmeister III at A=415.)

On the matter of price, 22,000 Euro (~$25K) is about the going rate for a new custom-built harpsichord, so that's not totally unreasonable. Other classical and orchestral instruments of professional quality frequently command similar sums, and new grand pianos easily get up into the six-figure range.

I would love to see one of these in person. Anyone spotted one in the US yet?