I live on the 6th floor of a walkup.
Plusses:
First year I lived here I went down two sizes.
I never feel like I have to join a gym or do any other exercise.
Doesn't get harder, really, although if I'm here for my elderly years I expect it will. (It does get harder with more than one trip in the same day, however.)
Cons:
If you ever get to the first floor and realize you forgot something upstairs, it's a pain in the ass. The only thing I'll go back upstairs for is my wallet.
Can't do a big grocery shop.
Some people won't visit.
One year I broke my arm and going up and down all those stairs was scary for the first month or so. I had to walk downstairs backwards so that I could grab the handrail with my good arm. I can't imagine what I would have done if I'd broken my leg. (Oh yea, and paramedics don't want to carry you down all those stairs either. I had to walk. ouch.)
In the summer, I have to make sure I have water with me for the walk up or I'll dehydrate by the time I reach my door.
Carrying pets up and down when they need vet care. (I think this is the biggest hassle on the list.)
An elderly neighbor of mine ended up pretty much floor-bound after an illness/injury.
Another post reminded me of this: The last ten minutes of any trip are always ruined by having to lug luggage up all the damn stairs.
Overall, I haven't minded it. I just want to move because it's getting too expensive.