We were originally really hoping to save our hardwood floors. They're original to the house, and in most places the wood itself wasn't in truly horrible condition. Unfortunately, one of the major structural problems we found ourselves needing to address was the fact that every single floor in the house was severely out of level -- to the point that when I sit in my desk chair in the office, I roll backwards constantly! If I don't catch myself or lock my foot around the desk, I'll find myself all the way across the room in relatively short order. It's like that everywhere. The upstairs landing in particular had a huge dip when we bought the house; this is what we found when we tore up the carpet:
The thing is, even though those floorboards look like they're in pretty good condition aside from the whole massive-tilt issue, they have to come up. There weren't any subfloors in the house (hence the subject matter for my last I don't even know how many posts). So to level the floors so I'll stop rolling around the house every time I try to get some work done, we have to install new, level joists and a carefully leveled subfloor.
All that said, we thought we could still save the floors and maybe carefully put them back over the new subfloor, with a little fiddling to eliminate the obviously warped ones. Unfortunately, when the demo crew went to pull up the boards, we discovered that they're so old, dry, and hard (the boards, not the guys) that they crack and splinter no matter how carefully they're pulled up.
So, new floors it is. Or will be, someday. For now, we live on plywood.