Not just pretty rare but virtually non-existent. Ever since the previous CEO, United has chosen to sell these upgrades to all and sundry on check-in rather than upgrade anyone who is not a global services member (which only United can create, and never would happen on a status match). You do get to check 3 bags for free on an international trip, which many United ticket/gate agents don't seem to know. I guess that's useful for some, but not for me as I do all trips with carry-on.