I’ve been to the US a number of times. I always book the Hilton room type that I’ll be happy with and work on the basis that upgrades may probably not happen. Most of these trips were pre-Aspire.
During my trip last month, the upgrades were at Hilton Whistler (to a larger room type with full kitchen) Canopy at Dallas Uptown, HGI Portland, Maine, (to a great water view, upgrade from the base room) and Hilton Palacio del Rio in San Antonio (to a Riverwalk view). No upgrades at Hilton LAX, Hilton BOS, Doubletree Kamloops, HGI YYC or WA Beverly Hills (but a great stay anyway). So, all up, there were probably more upgrades this time than my average trip, for whatever reason.
As an aside, I also stayed at some non-Hiltons in the US and Canada, including the TWA Hotel JFK, just for the experience, and Fairmonts in Banff, Lake Louise and Jasper. No upgrades at any of those of course, except for an unexpected upgrade at Jasper to a full lake-view Junior Suite.