My mother-in-law is about to get a knee replacement. She's 82 and extraordinarily active, but she's been hobbled the last year because it's just bone on bone grinding now. She's been bugging her doctor about getting the surgery for months but he's been reluctant, wanting to make sure she was really at that pain threshold before going ahead.
A brother-in-law had it done about ten years ago. He's a giant guy, former offensive lineman, something like 6'7 and 325 lbs, super outdoorsy guy. The surgery did not go well and I don't know all the particulars. He got some sort of blood infection, almost killed him. His life has been ten years of hell - he's been reduced to occasionally shuffling around, uses a cane on good days, never gets out of the house, his heart is all fucked up and he's a rollercoaster of maladies and hospital visits. Horrible.