Windows 8.1 with KB3000850 has been working perfectly for me for 3 days since removing Avast.
It's a big update... As was mentioned above, it's basically a service pack without any change to the version number.
The question is this: Why wouldn't you want the latest update from Microsoft? You have been trusting them up to this point to deliver updates to you that work. Those of you considering not installing it - it's your choice, certainly, but moving forward, aren't you going to wonder whether you're falling out of the mainstream? When new applications are developed, or even subsequent Windows Updates, do you think they'll have been better tested on a FULLY up to date system, or one where KB3000850 was left off?
The problem with this update has been with Avast, not with Microsoft.
-Noel