Legion of the Damned by William C. Dietz
In the future criminals and the terminally ill all have two choices, be executed or join up for the Legion of the Damned These fighters are cyborgs soldiers and humans who have had their bodies changed, and their minds put into huge war machines that will fight off the enemy. This is the first in the series that mixes military science fiction and adventure. It is this series which incidentally started the Andromeda prequel series of novels.
