I hadn't, until reader M. C. sent me the link to the video clip below. According to Wikipedia, an otamatone is:
"an electronic musical instrument synthesizer ... It is a singing toy whose body is shaped like an eighth note (it also somewhat resembles a tadpole, "otamajakushi" being Japanese for "tadpole"), with sound emerging from a "mouth" on the notehead. It requires two hands to play: while one hand holds and squeezes the "head", the other hand controls the pitch of the tune by placing the finger on a ribbon controller on the stem; a higher position on the stem creates a lower sound. The ribbon controller is deliberately delinearized to resemble a guitar, so there is a shorter distance between higher notes than between lower ones. Varying the pressure on the head (thereby opening and closing the "mouth" of the instrument) creates a wah-wah effect, and shaking the neck (and thereby slightly changing pressure on the head) creates a vibrato effect. Switches on the back of the head allow users to change octave, turn it off or on, or change the volume."
Here's the theme from the Pink Panther movies, played on three of these weird-looking beasties.
Personally, they remind me of Denis Norden's (in)famous response when asked what was his favorite musical instrument: "Bagpipes - receding into the distance!" I suspect otamatones might be similarly improved . . .