I am running 8.1 in developers preview on my first Windows Phone, a Lumia 1520, after many years on Blackberries. One of the still-truly-great things about Blackberry is you can install and run "leaks" and/or other carriers' official OS upgrades on your devices. Blackberry continually tweaks and improves their OS and the carriers in the US were so appallingly slow to approve and rollout new updates that the ability to run leaks or betas became an absolute positive for me. Some US carriers were literally months or even years behind the curve of the latest upgrades. So the ability to nonetheless remain current with the latest OS became critical for me, particularly when BB10 allowed us to run Android Apps. So I've had a lot of experience with upgrading to betas, sometimes downgrading when they were too buggy, and generally assessing the merits of an upgrade by comparing the added values of the new system against the bugs and apps rendered inoperable.
On that plus vs. minus basis, 8.1 is hugely meritorious and among the most stable and bug-free upgrades I've encountered. I have about 150 apps installed and only 5 that are now no longer available after upgrading to 8.1 and 3 of these are available as newer versions. I had an initial issue with battery performance, which settled down after some tweaking and time, and has been resolved. I have had a few instances of dropped wifi which has been easily correctable. Essentially no freezes and only a handful at most spontaneous browser related reboots. All of this despite not yet having the Lumia firmware upgrade and with almost ceaseless tweaking and installing and uninstalling, etc., as per the usual shakedown cruise under a new OS.