I recently tested 3 guitars.
The first was a Fender Strat which had just had the frets dressed and the earvana system installed.
The second was a Washburn that was new with a factory Buzz Feiten system.
And the third was a Carvin that came with a graphite nut.
All guitars had great fret work, which is paramount to any tuning system. If your frets are not level and crowned, no tuning system will work. Finger pressure and fret size also make a world of difference. If you have a heavy hand and tall frets, you may have to make some adjustments if tuning is a problem. These guitars were checked with a peterson tuner.
So here goes,
The strat with the earvana played in tune very well, and was every bit as good as the Buzz system. The only downside(if you can call it that), was the material of the earvana nut. It is not as tone conducive as bone or graphite, but it is not bad either. The plus side to the earvana system is the cost, and it requires no special setup. Just tune it and intonate it like a normal guitar.
So the Earvana gets a 9 out of 10.
The Washburn with the Buzz system was great, and also was very accurate, tuning wise. The only complaints regarding the Buzz System is the price, and the repairman aspect of the deal. Most Feiten guys will want to dress your frets before installing the system, unless your guitar is new. It can be a bit pricey, depending on the shop, and not all Feiten guys are good luthiers. I had a guitar years back that was destroyed by an authorized Buzz Feiten repairman. I got my money back, but the guitar was hopelessly destroyed.
The tuning was consistent over most of the neck, though not perfect. No system can compensate for things like finger pressure, fret height, etc.
The Buzz gets a 8 out of 10, due to the price and unpredictability of guitar repairmen.
And finally, the Carvin with the graphite nut was IMHO superior in the middle and upper registers. Maybe because the Carvin was better built, but to my ears and the tuner, the Carvin was superior on octaves, and triad chords throughout the neck. This is not to say that the Buzz and the earvana were crappy, they were in fact very good. But to me, a well cut nut and good setup are more than enough.
The Carvin gets a 9.5
Just my thoughts.