I turned it on a week ago to see what it was like. I expected it to be significantly annoying, but I found basically nothing changed other than a bit of text in safari that says it's in lockdown mode. Otherwise I wouldn't have been able to to tell the modes apart. I was expecting the browser to be slower without JIT or use more battery but I haven't noticed any change, it's all still snappy.

Apple over hypes the "you need to be in significant danger" part. Basically anyone can turn this on and it's fine. The UX seems mostly exactly the same either way.