IMHO, the Earvana nut does exactly what it says it will do. One of the most frustrating aspects of my early guitar learning was, let's say, tuning a 1st-position Emaj chord to perfection, then strumming a Dmaj and having it sound like a warped record. Then, I'd correct the Dmaj, and the E would sound horrible. One would constantly tweak the G string to try and find an OK compromise. With the Earvana, problem solved instantly.
Now, of course, nothing in life is for free. I have found that the Earvana can cause some slight pitch problems with partial chords higher up the neck (e.g., the opening riff of "You Don't Have To Be Old To Be Wise" comes to mind right now). If you do a lot (and I do mean a lot) of chording higher up the neck like that, the Earvana may not be for you. It's not a huge problem, but it is noticeable. I've read that some jazz purists don't like it, but I've never been able to quite decipher their admittedly very musically sophisticated comments as to why (being neither a jazzophile, nor much of a purist at anything).
For me, however, it really turned my playing around since I had my first one installed (I guess about 5-6 years ago), and one has made its way onto every guitar I own. I've yet to personally meet anybody who actually tried one who didn't like it. I think that it's one of the best bang-for-the-buck (pound?) tonal improvements that one can make on a guitar. (With the exception of BKP's, of course.)
cjpm
No, I own no Earvana stock, take no kickbacks, etc. :)
