Since Radiohead started using it I've known the Ondes Martenot was a very cool instrument, that it's rare, and that it was used by the composer Olivier Messaien. But I never realized how it worked, and just how ridiculously cool it is, until I watched this video demonstration by one of its foremost experts.
The Martenot has a keyboard that produces an ethereal sound that wouldn't be out of place on a modern synthesizer, and the timbre can be modified with some controls similar to the stops on an organ. The keyboard adds an extra dimension of expressiveness with vibrato that works by simply nudging the entire keyboard back and forth with the finger you're using to play the note!
Then there's the feature Jonny Greenwood loves, a string that runs the length of the keyboard just in front of the keys, with a metal loop that goes around the player's finger to pull the string back and forth. This produces a smooth tone with continuous variation in pitch.
I'm kind of amazed that this feature hasn't been incorporated into more modern instruments. Portamento is probably seen as the substitute, but there the glide between pitches, while customizable, is basically the same on every note transition.
This leads me into an observation I've been mulling over about instrument design. Modern instruments, from synthesizers to drum machines to laptops to circuit-bent toys, offer us plenty of novel sounds and compositional techniques. But for that they seem to trade in the expressive power of classic instruments. When I play the guitar, piano or drums, it's very natural for me to match my playing to the mood of each moment in the song and introduce variations that add interest. The actions of my fingers are intimately connected to the variations in sound.
With a synthesizer or a drum machine the best I can hope for is that the timbre as well as the volume will be affected by the velocity with which I play a note. Further control is only offered by knobs, sliders or the pitch wheel, a less direct connection to the actual playing of the instrument. On a laptop I program a beat that by default will be exactly the same throughout a song, and I add variations using a mouse, a far more laborious process than simply playing it the way I want it to sound.
Much recent software development, such as some plugin effects by Audio Damage, is intended to counter the monotony of programmed beats in loop-based music with randomized variations. You can request a reverse playback, or a stutter, or a repeat of the snare hit 33% of the time, and you can achieve some very neat sounds this way. But randomization is never going to sound the same as a talented human musician's reaction to a song and to other musicians.
What tends to happen with circuit-bent toys and other experimental instruments is that I figure out quickly the one or two sounds they can produce and I find a song to use it on. It's not worth my time trying to master it the way I would study the guitar, because there's no mastery to be had. And ultimately single-use devices are not going to be worth the space they take up.
I think this is also why I've had so much trouble getting anywhere with ChucK, a music programming language. Periodically I get very excited about it, start playing around, and end up with yet another random melody generator. The language is extremely well designed and powerful, but programming is just a very remote, cerebral interface to music. Or to put it another way, generative music, while conceptually appealing to me, is hard to do well.
Lastly, this ties in to why people often find electronic music performances boring. I think at a primitive level we like to be able to visually connect the musician's movements the sounds being produced. We can only guess at what is being accomplished with knob-twiddling and a mouse.
To sum up, while I always applaud inventors of these instruments, software and other devices, I think we would do well to remember what has allowed the classics to endure for so long. I am of course happy to hear about counterexamples that have offered great expressive power. And come on, let's build a new Martenot already!