The Theremax is a way to control Voltage-controlled equipment without a huge outlay of $$ -- and I would add that it performs well as a controller.
The tone quality can be improved with a couple of minor modifications and I am very pleased with the Tmax's timbre. I wouldn't say that the instrument is unplayable -- I am able to play music on it.
The instrument seems sensitive to external electrical interference from radio stations, etc... which is noticeable below middle-C. The instrument is too noisy for professional recording unless one resorts to EQ and noise gating.
Since this is the only Theremin that I have played, I may be making music despite its limitations. I am taking delivery of an Etherwave Pro today or tommorrow and I expect that it will be much better than the Theremax -- then again, it is about six times as expensive!
I agree that if one's motive is to play music (rather than tinker with electronics), the Etherwave is a better way to go... when dividing the cost difference between the Tmax and Etherwave by the hours of constructing/tinkering it works out (for me) to around $3 per hour. (The pro comes to about $20 per hour, btw).
At the end of the day -- I was skittish about laying down the $$ for an Epro without even knowing if I'd enjoy playing the Theremin other than using it to control my synth. The Tmax provided many hours of enjoyment because I like building kits, I learned A LOT about Theremin theory of operation and circuits, and discovered that the Theremin is a BLAST to play. My 8 year old son helped me build the Tmax, thus the Tmax will always carry sentimental value -- and will likely continue as a controller for my modular synth because it has a few CV out's that the Epro does not.