Haven't yet and to be honest no intention.
I ALWAYS build new and migrate the Domains across one at a time.
Unless you actually need an R80.40 specific feature then no need to yet.
Will be waiting for the Domain Migration ability to be backported to R80.20 so that can export the Domains one at a time.
Still a bit early and R80.30 still recommended.
Is a bit Chicken and Egg,
People won't update till is recommended and won't be recommended tilll more people updated.
If I was doing a NEW Clean install then would do R80.40 but otherwise not just yet, and I suspect most people here will be the same.
I do intend to upgrade my Lab however as inplace upgrade so when ready to do that will try and find the thread and report back