Use of a super tweeter worth the investement ?

Good question. My speakers have a horn in the center and a 12" bass driver above and below the horn. I tried it on top of the speaker at first as Townshend recommends, but I suspected that  it would be better to try to align it with the horn, so I built a simple pair of wooden stands that were the height of the horn, and then in an attempt to time align them, I moved them back a bit farther than the distance from the mouth of the horn to the dome of the compression driver in the horn. The result was immediately noticeable.

I think that Townshend and their dealers (one of whom I spoke with) should amend their placement recommendation and encourage users to experiment with placement. On top of the speaker might be good for some if the top of the speaker is at ear level and the tweeter of the main speaker is near to the top, but in my case, the top of the speaker is far above my ear level, and if you're using a ribbon supertweeter like the Townshend, you need to consider that they don't have wide dispersion like dome tweeters, so they should be at or near the level of your ears and pointed toward you. 

I recently added a pair of Elac 4pi's and wow.

Like many have said above, re sound stage opening up, tighter bass, much more recognizable detail in all frequencies. I would not want to take them out and actually added a second pair to an office system. I really do feel the super tweeters just upgraded my main floor standers to punch way above their weight and sound like 5x their cost. They weren't cheap but they did add their value and more, imo.

I inserted Taket Batpro 2 supertweeters in my main system with my Daedalus Argos V2s. As others have said, they have a positive SQ audible effect across the frequency range. I have them at the front edge of the top if the cabinet centered. I’d suggest that you research ones that allow you to adjust their sensitivity range to align along the line of your main speaker's sensitivity

@emaillists - the Enigmacoustics Sopranino is still available

emaillists,

I enjoy the Enigmacoustics Sopranino with my Audiostatic 240 electrostatic main speaker.  I use the low crossover setting at 8 kHz, which greatly increases the output into much lower freq, compared to the high 12 kHz setting.  Even though the crossover slope is 12 dB/octave, there is quite significant output at midrange freq.  So the benefits of the Enigma are not due to more HF extension, but to more emphasis of upper midrange overtones.  That's why bass instruments have more clarity--more overtones are revealed, displaying the bite of the instrument.  

The only caveat is the parallel connection to the main speaker, which will greatly reduce the net impedance of the combination of the main speaker + Enigma.  Stereophile had informative measurements of the Enigma impedance at the 8, 10, 12 kHz settings.  At 8 kHz, the impedance dips as low as 1.75 ohms.  At 12 kHz, the impedance is about 4 ohms.  If the amp has a weak power supply, the output will be much lower at very low impedances.

Time alignment of the Enigma with the main speaker is important.  Align the driver in the throat of the Enigma with the tweeter throat of the main speaker.  The front of the horn of the Enigma will be quite forward compared to the main speaker.  There is a big improvement in clarity with proper time alignment.