This instrument is at least as hard to play as a violin!
Sergei Rachmaninovs Vocalise, played by Peter Pringle on his 'Goldie', the recently restored theremin, built around 1929, that belonged to Mr. George Goldberg. Goldberg was the assistant of Leon Theremin during his tours through Russia, Europe and the USA. He lived in the USA untill his death.
Source: Theremin.info