The offsets from 12TET are 4 cents or less, which is no larger than unavoidable pitch variations caused when playing a guitar.
4 cents tuning error only causes a moderate pleasant chorus effect.
However this does mean that a TT guitar will vary in pitch from other 12TET instruments slightly more than a normal guitar.
So unless you are going to play chords on a bass i do not see much point in TT, unless you are one of the few musicians who actually want the 'key colour' feature of a Well Tempered instrument (due to the unequal sizes of the 12 semitones in an octave, a particular scale will have a slightly different character in each possible key)
Thought i should back up my last post with information from the TT website.
I get the impression they are scared that the truth will put people off, and i can see why, so the basic up-front dumbed-down marketing is "Perfect pitch / perfect intonation / in tune, all over the fretboard" which is misleading and deceptive. This marketing takes advantage of the fact that many people incorrectly come to this conclusion from hearing the improved harmony of chords. Chords in 12TET inherently have poor harmony, so you cannot have perfect chord harmony and perfect 12TET at the same time.
Strandberg repeat this marketing on their site as they are working with TT now.
Then more hidden away on the TT website is the details:
How to tune
"How do I tune?
[...] the open strings are not tuned in equal tempered intervals.
The Thidell Formula One temperament is built around fret positions and tuning offsets."
"What happens when True Temperament guitars are used with other guitars, bass, piano etc?
It works just fine together with “ordinary” instruments. The offsets from Equal Temper are not so severe that they create any dissonance whatsoever."
"True Temperament offsets info
Cromatic offsets in cents from equal temperament as follows:
E -2, F 0, F” -4, G +4, G” -4, A 0, Bb -4, B -1, C +2, C”-4, D +2, Eb -4."