It was fine. In NYC roommates and I have been in different apartments that were both 5 and 6 floor walkups. This was when we were in our 20's - early 40's. Thing is that after a few weeks you don't really notice the stairs anymore. Bonus is your legs will be pretty strong!
Sure we also had to lug groceries/laundry up the same flight of stairs, a bit annoying but nothing unusual. Didn't have a car either so all that stuff would get carried x amount of blocks from the apartment or even a subway trip.
In my late 40's / going into 50's I'm not so sure I'd still do those type of walkups anymore.
PS - Yeah pay for movers when doing moves in/out of walkup buildings, you really don't want to do that yourself. It's fine when you only need to go up the stairs once/twice a day but repeatedly for a move is much harder.
Oof, yeah I've seen buildings like that. In NYC in the Lower East Side inside some of the older buildings you walk up one flight of stairs then walk the hallway across the length of the building to the other flight of stairs. That sort of setup feels so much worse vs just toughing it out up a column of stairs.