Sort of. The complication is that metal doesn't flex much at all, whereas bone is much more flexible and shock absorbing than you think. Replace a leg bone with metal, and you'll feel it in every bone of your back with every step.

Hip and knee replacements are a mature technology. They typically use titanium, because bone will naturally graft itself to the metal, making a very solid connection. But it's a surgery that's almost routine at this point.

Essentially, they cut off the "ball" end of the femur, and replace it with a titanium ball (YouTube has some great videos, though they're not for the squeamish.) The socket part has a hemispherical insert that gets screwed in place. There's a plastic lining between the two, so you don't have a metal ball rubbing directly against a metal socket -- your body naturally creates cushioning membranes at the joints that provide well lubricated, almost friction-free surfaces, so the manmade joint needs its own, too.

There are complications. Typically, because it's an unyielding metal joint, they don't want you doing high-impact things like running or jumping. Because it's a major surgery, there are the risks of infection, pulmonary issues, deep vein thrombosis, and you'll be in massive pain afterward until the surgical incisions have healed, and the supporting tissue around the joint has healed. (This part is non-trivial -- it's a metric boat-load of pain.) And perhaps not surprisingly, doctors don't always get the length right: a known risk is that you'll have two different-length legs.

Here's a good page from the American Academy of Orthopedic Surgeons. http://orthoinfo.aaos.org/topic.cfm?topic=A00377

Usually with the spine, though, they prefer to fuse vertebra together, or graft on bone tissue (typically from the patient's pelvis) to "shore up" the damaged vertebra. That's much more tricky, because the spine does so much more than leg joints, and it protects the precious spinal column.