Comment Fighter pilots (Score 1, Informative) 50
But I don't think people with normal abilities will be trading in their limbs just to be able walk a little longer, run a little faster, or carry more weight.
You might see fighter pilots getting this done in order to avoid blacking out at high g-forces when the blood drains out into the legs. Examples include Sir Douglas Bader, who rejoined the RAF after losing his legs in an accident, and Super NES-era Fox McCloud, who is depicted in an illustration on the cover of Nintendo Power as having metal legs.