Depends where you are from and your tolerance to latency.
In PvE, depending on what you're doing and with whom you can still play good enough (get the boss down) with 500ms be it as a tank a healer or a Damage dealer. So it's playable but not so fun. Now lower than that can be acceptable though since you can't trigger the next combo step or anything before server recieved the info on the previous step, having latency slows everything down.
In PvP you can't do good with high latency in a competitive envrionment. Double taping is limited, hitting moving target is based on luck and guesses. Reaction to CC, smoothness of action is gimped. So sure, i guess you can "do fine" when you've been playing like that for ever. But "doing fine" is not ok when you were use to do better. And it's not just a question of stats at the end of a mini, it's also a question of feel.