These instruments have VERY SLOW RESPONSE TIME - the produced sound lags way behind the finger stroke. This renders it beyond useless for either private practice or playing in sync with other musicians - and since these are really the only two uses for the app, it's of no value.
Don't waste your money.
Have not tried the other instruments in the set but one can assume the problems are similar.
That being said, it is nicely designed, and the sound samples themselves are reasonable. Hopefully latency time can be considerably reduced in future versions.