I completely agree with Jace-BeOS, the quality of this program is hard to believe.
The sound is as dynamic as the one of the original instrument. The way it reacts to changing the stroke direction is very realistic, there's no way to do this with samples. The specific quality of each of the four string instruments is preserved very well: four players with four iPads (and four amplifiers) could do a very good impersonation of a string quartet.
At the same time the interface is easier to play than the original, and it's very configurable too: the pitch note is correct wherever your fingers land (as if having frets like on a guitar), but you can disable this feature if you want even more realism.
Another very welcome setting changes the pitch of each string. I set the intervals between strings to fourths instead of the original fifths, so that the fingering is closer to that of a guitar or bass.
Something very hard to do on the original: you can play three or four strings at the same time, not just one or two. :-).
As good as this program is, you can only play it via the touchscreen: it has no MIDI input. Driving it from my LinnStrument would be awesome. Matthias, what do you say? :-).
FingerFiddle is one of the best music instrument applications I've ever purchased on any platform, especially considering its low cost in comparison to its high quality.
It's the only physically-modeled bowed instrument software that has a natural-feeling performance GUI, and that's largely due to being developed for iOS and touch screen interaction (the larger the screen, the better, and I recommend it on an iPad Pro 12.9"). Playing FingerFiddle is leaps and bounds beyond any other bowed instrument software on any platform that I've tried. It even simulates pressure by the rarely utilized touch-surface-area feature of iOS touch screen devices (the harder you press on the screen, the larger the contact area is, as your finger squishes out onto the glass).
The developer has always been responsive to my inquiries.
It's a shame so few people seem to know about or have interest in it.