I rode it with three other people two years ago from Pittsburgh to Cumberland. Two of us felt like the uphill was almost imperceptible most of the time. One of us noticed, but didn't seem to mind for the first two days at least. One of us was miserable and spent a fair amount of time at the bike shop in Confluence considering other bicycle options, including electric, and the next time we went riding together, he did have an electric bike.
On a previous trip I met a guy with a fully loaded bike in Meyersdale who had come by way of the C & O, so was going from Cumberland northwest. He had nothing good to say about the trip between Cumberland and the continental divide. Too bad, really, because it has some excellent views and is actually one of the more scenic stretches, I thought, but I guess it's easier to apreciate all of that when you're coasting.