Well, what's to say we would have to outright replace our limbs at all? For example, the HAL (Hybrid Assisted Limbs) from Cyberdyne doesn't need to replace your limbs and instead just assists them. Things like the HAL are working great so far, and have uses in medical, and non-medical applications, for example:
Non-Medical Uses
Medical Uses
http://www.cyberdyne.jp/english/products...dical.html
You can see above clearly that HAL can help many people with injuries be capable of walking again.
Now, then we have full limb replacements with robotics. Those are still in very early stages right now, but I think with the rate technology is advancing, I'll be shocked if they aren't fully developed within our lifetimes. Now, I wouldn't outright replace my limbs with robotic parts instantly, however, it's definitely something I'd consider depending on the capabilities and reliability of the robotic limbs when they did arrive, if they ever reached a point where they were reliable, could greatly increase your life span and possibly even improved what you are capable of doing such as with HAL, then I would seriously consider it, and if I had some sort of terminal illness or was old and dying anyway, I would switch to it in a heartbeat.