I have a few guitars with evertune.
First and foremost, anyone who buys and evertune guitar should do as much reading about setup as you can. The number one reason I see people complain about evertune bridges is because they do not know how to set it up correctly for what they are trying to accomplish.
As for sound, sustain, etc.... You can hear a slight difference in tone, but no more than going from a hard tail to a trem. My evertune equipped guitars sustain just as well as my non evertune guitars. The tone is just as thick and chunky, and the guitars play just as well as any of my other guitars. Full ability to bend, while also keeping the guitar perfectly in tune. My evertune guitars are such a joy to track with because they literally NEVER come out of tune.