It's a cool device, but to me it's a solution looking for a problem. On fixed bridge guitars tuning stability with a properly cut nut and healthy bridge saddles is usually not so bad to be a major issue requiring something as complex as Evertune.
It's not a subsitute for a Floyd Rose trem, since Evertune is not a trem you can use to alter pitch afaik.
I consider the Evertune to the fixed bridge as the Floyd is to the trem. Neither are necessary and yep, a well set-up guitar will work fine as we've all witnessed for decades. At this point the only reason I want another Floyd guitar is just because I like Floyds, I can go crazy on my Strat trem all day with the same tuning consistency as a Floyd, it's just a different feel.
But the Evertune is a bit different, because while I might still need to tune up occasionally with a Floyd, you don't
at all with the Evertune.
This mostly comes into play for me when layering guitars. Someone argued with me the other day that you can't hear .1/.2/.3 cents of difference when layering guitars and I vehemently disagree. It's not going to sound like an out of tune guitar, but you're going to start hearing some of that chorus-like sheen coming in and it drives me fucking nuts. Eliminating that is worth the price of admission alone. Even a perfectly setup guitar is going to slip a couple cents, especially with a heavy handed player. The Evertune doesn't slip at all.
They don't mention it much and I feel they should, but it also helps with intonation all over the neck. I recorded this this week as an example; I haven't touched that guitar in 6 months, didn't check the tuning, just dialed in the most unpleasing "You're gonna hear the beating" tone I could. The first part is just maj/minor chords then the end is multiple octaves all being played at the same time, shifting around the neck....all wonderfully in tune.
And because my Les Paul slipped when tracking rhythms a couple weeks ago, I'm debating re-recording all the guitars in this. You really only hear it when I play chords, but it's enough of that chorus effect to drive me nuts.
If I tracked that with the Evertune guitar, it wouldn't be an issue.....and I wouldn't have checked my tuning 50000x between takes only to still have it come out botched!