Should be noted that any ensemble that consists only of continuous-pitch instruments (eg. strings, trombone, human voice, fretless guitar) is free from the shackles of 12TET and its harmonies naturally tune to as close to "pure" intervals as the performers’ skills allow.
Naturally JI instruments include any instrument which has a continuous frequency domain, like trombone, fretless stringed instruments, and human voice.
Instruments with keys or frets may be "bendable". However, the direction and quantity of bend varies with with each tone it is paired with, making this incredibly impractical.
It seems to depend a lot on their age and experience. Years of only single 12TET will tend to pull them away from the JI "sound". However, I know a few choral singers who've tried (in midlife) to single JI and once they got over the initial "hump" they found it quite natural.
The "beating" that occurs in 12TET is inaudible to most western listeners because we've become so acculturated to it. You have to get some exposure to music where it doesn't happen before you even notice it (and thus seek out more consonant harmony).
