Intonation is a problem for classical guitars if you think about it to much and guitars in general always seem to be out of tune or straying that way. As my guitar teacher said at a concert. “Guitarist spend half their lives tuning their guitars and the other half playing out of tune.” Maybe there’s a solution, guitar necks that have supposedly true intonation necks, no more intervals and notes being slightly flat or sharp.
True Temperament necks promise to produce a guitar where all the notes on the finger board are actually in tune. Pictured left is a True Temperament neck on a Alhambra Cutaway. Looking closely the fret on the G string is “Flat” and the second fret is virtually straight. This would solve a problem I always had tuning guitars in different keys. If I tune the g string to be perfectly in tune (say with a g on the top E) it sounds sharp and creates an ugly dissonance if I play an E chord in first position.
If I then flatten the G string to sound in tune to the E chord the G open string sounds horrible in a open C chord. I’ve always either compromised or tuned the guitar to suite a particular key. Flat G for pieces in E Major and in tune G for C Major. This second neck has a “flat” G# so it would sound in tune in a E Major and C Major chords :).
The key question is “how does it sound” or more to the point “does it sound better”. The video demonstration doesn’t answer the questions. It sounds nice but I can’t compare it to the same guitar with a normal neck. Supposedly the chords ring out longer but who knows?
The real test would be to fit a True Temperament neck on an existing high quality classical guitar, but I can’t see anyone volunteering to do that. Another way would be to get a new quality guitar built with a True Temperament neck and compare it the same guitar model built by the same guitar maker at the same time.
My other concern is “how does it play”. Those bent frets seem daunting to me. Some claim this wouldn’t be a problem as “you don’t place your fingers right up against the frets anyway” but arr I actually do, that’s the way I was taught. When I read music I feel for the right position by touching the frets. A fingerboard with frets in the “wrong” position would throw me at first. I’d probably get over it with careful slow practice.
Other reservations from forums state that a True Temperament neck would only work in certain keys and sound out of tune in some popular jazz keys.
There are some interesting resources on the True Temperament website:
My old spruce guitar 
Standard fretting is calculated on the basis of the scale length only. This approach may cause the intonation to be a little imprecise but, at the same time, it has a very big advantage: the fretting is definitive in the sense that you have no reason to change it during the whole guitar’s life. On the contrary, the True Temperament Fretting System takes into account many factors to construct guitar fingerboards. But all of these additional factors may change during the guitar’s life. So, I wonder what does it happens if one decides to adjust the guitar action or even if one opts for different strings type or brand. Should one change the guitar neck too?
No intonation system is perfect. That’s just how physics works. New strings will intonate differently than older strings; the amount of pressure you apply when you play changes your intonation (more with bigger frets, but this still applies to shorter frets, too), etc. These are closer to perfect from the start, that’s all. What additional factors are you talking about that change during a guitars life, that don’t also affect guitars with normal frets?