I have argued in favor of mechanical life (see my blog of 19 Oct. 2010). This can be engineered rather than an evolutionary kludge. (There would be no vestigial organs, etc.) In addition to repairing components or "melting them down," the fusion torch (Eastlund and Gough, WASH 1132, 15 May 1969) would create a fully closed matter cycle; mechanical life need not input new matter ("food") nor output any waste matter. In addition to chemical energy mechanical life could make use of nuclear (or other) energy, performing larger amounts of work than is possible with present-day ("natural") biological organisms. Mechanical life forms would be super organisms.
If space aliens are capable of traveling to earth we might expect them to be of this sort. (Because of the high energy demands for relativistic travel I assume these should be as small/light as possible.)