Craig (from headfi, Music to My Ears) and I spent two hours last Sunday at the cave comparing Tidal masters to the original recordings (both at Tidal and at Apple music) over various cans (Stax cans rule) and through the speakers (we used Fleetwood Mac Rumors as reference tracks, Audio.gd DAC 19). Tidal masters sounded better both in terms of noise and some spatial sense that they somehow add (maybe using reverberation). The noise floor was definitely lower, as if the file had been DSP processed the subtract noise. The notes (cymbals etc.) seem to linger a bit longer. Overall I liked it. Did I like it enough to justify the $20/month? That is a tough subjective opinion (to me the great value-add by Tidal is the music videos, but this masters thing is not bad), but it is a nice feature.
(Now I am going to click on the link Carl posted to see what the rest of the web thinks of Tidal masters)