Looks like an interesting way to record a piano. Anyone played with one of these things? (Not a toy--They list Berklee College of Musis, Paul McCartney, and more as clients using this--basically a set of pickups for a piano.)
I spent the 80's decade touring with Nashville stars, and dealt with micing pianos on stage every day for ten years. As Lee said, the Helpinstill has a strong, almost "electric guitar" sound, so it's useful in a loud band situation for volume without feedback, but it's far from natural sounding, and there were always "hot spots", (notes that jumped out too much)
In the late 80's "C-tapes" by C-ducer became popular, and they were far more natural sounding. They fastened onto the underside of the soundboard with doubled-sided tape, or could be taped on with gaffer's tape. It usually took me 30-45 minutes to find the right spots for the tapes on each piano, but with the right placement and some eq, they sounded pretty good. We played a lot of "Pops" concerts with orchestras all over the U.S. and Canada, so I would mark the tape position on each piano, and when we came back the next year it would save me an hour of experimenting. So all over N. America, there are grand pianos in the symphony halls with little sharpie lines and my initials on the bottom of the soundboard!
Even with C-tapes, we still used mics on the piano, and our FOH engineer would use the mics as much as possible, particularly on ballads and piano solos, and bring in the c-tape channels for the louder sections.
To be honest, the pick-ups were always a band-aid for an acoustic problem, and I can't think of a reason to use them for recording/sampling a piano, unless of course, you want to make it sound like something other than a piano-
I've used stuff like that when using a real piano on stage with a loud blues or rock band. Mainly because it's so hard to mic a piano and get it loud enough to be heard without feedback. I used a pair of Barcus Berrys which seem to have reasonably good sound quality. But these days I'd be more likely to solve the problem with a midi output to gs3 piano sampler using a piano bar or something.
I seem to recall Michiel Post mentioning a while back that he was experimenting with piezo pickups in pianos. Sticking them to the soundboard as oposed to mounting them in the bridge assembly, I believe.
I've also heard rumors some folks are experimenting with sticking miniature omnis onto piano soundboards and bridges for sampling with spectacular results. There was mention in last months Electronic Musician that hinted at that type of application in a review of the DPA IMK4061.