Lumia 920 here.
Did the update, and immediately an Hard Reset after that (like I always do on any major release, to squash bugs), and after some days of testing:
1) After a little interface lag at the beginning, now everything is faster and smooher then ever before. My 920 never was this snappy and fluid, even apps open quickier;
2) All notifications are now real-time, and all the past syncing problems are gone;
3) 4 rows tiles (with Medium and Wide tiles, cause the Small ones are too much small for me) with parallax background is pure eyes sex. And everything is even more "Live" and dynamic;
4) Apps auto-updating are great;
5) Action Center is now the best notification center on any platform. Just so clean and pefect;
6) WordFlow is golden;
7) IE11 is so much faster then IE10, and new Reading mode is nice
8) My battery drained quickier on the first day, but after a full discharge and full recharge just after that (without touching the phone), now it last even longer then it did on 8.0 GDR3 (with so much more Background Tasks disabled).
8.1) NO bugs and problems at all for me. And I'm sure that with Nokia Cyan and new firmware/drivers everything will be even better on July.
This is truly the best Mobile OS ever made and I ever had so far.
I highly recommend everyone to upgrade if they can.