I had both knees totally replaced (the rubbing parts sawn off and replaced with plastic or something) in 2010 and 2011. Perfect results, both knees. Perfect mobility, range of movement and no pain at all. I got the impression that the op. won't be offered if you are too young, say below 75 or so, unless the problem is really bad. This seems to apply to most major operations. I think the reason is that they could go wrong and its better to wait until the relief to be gained from the op. is as beneficial as the continuing disability is bad, so that the two balance, more or less. In other words, it would be silly to have a major op. just to cure a twinge.
Another factor with joint replacement is that they last for up to 15 years, and although a second replacement can be done, again it would be silly for a 40-year-old to have the first op. and face another one at 55 and then another at 70. I was walking on crutches an hour after each op. If you think about it, the replaced knee is now perfect and can therefore take the body weight immediately. The only reason for crutches is that the leg muscles need time to recover (about 6 week until normal use is regained). The knee op takes between 45 minutes and 1 1/4 hour.
Hip replacements are very successful (my wife had one about 10 years ago, also very successful), with knees just behind.
If I could have my life over again I would be an orthopaedic surgeon (or a world-famous opera singer).