I have used the first version of the Earvana nut and have to say it is quite obvious that it does what it says it does, .....the catch is...if your the only one in the party your jamming with that has the Earvana, your going to be slightly different in pitch than the rest of the strings in the band, yes you will be the one with the truest chord tones and scale runs etc. but if everyone else sounds equally in key but you sound different, then whos right and whos wrong???
Im not saying its going to be so blatantly obvious wailing at high volumes but it will be noticable to you after a while, I decided to quit using the Earvana when I realised it changed how unison bending and certain chords sounded when jamming to Stevie Ray records, he does not use an Earvana nut, and I started noticing some of the licks I usually jammed along with, sounded different than when I had a traditional nut on my guitar and played the same licks.
I actually thought the Earvana thing would catch on with more people as it really does sound noticably better or more in tune, but since, I have realised its unlikely I will run into many guys using the Earvana to jam with, and much more likely that I will end up jamming with guys who have a traditional nut, so rather than be the odd guy out, I opted to use a traditional nut and really fine tune my setups, with optimal nut slotting, very precise fret leveling and action adjustments, and really accurate saddle intonation with a good strobe tuner. Now my guitar sounds so surprisingly sweet I cant believe I ever thought I needed an Earvana Nut to begin with.
If I were to use an Earvana Nut on a nice Warmoth neck I would definantly get a model that dosn't require altering the traditional nut slot, that way you always have the option to go back to a traditional nut if you decide it dosn't fit your needs, and you wont have to buy a new neck to do it.