In the future, there are human colonies out in space run by the Colonial Defense Force. The CDF keeps many secrets from Earth, including their skip drive and information about their colonies and wars. The US has nuked large portions of the Earth and people who live in those portions get to go be colonists. The only way USans can get out there is to sign up for CDF service when they're 65 and go when they're 75. Everybody assumes they'll get enhanced in some way, since most 75-year-olds are not good warriors. John Perry, our protagonist, heads up the space elevator (without a counterweight) to join the CDF. He finds himself in a new body -- green and cat-eyed -- and then under a drill sergeant's thumb.
Perry turns out to be a good soldier, with original thinking, and is rapidly promoted. He always has all the answers and knows how to do things. This is the Marty Stu part.
There are horrendous battles, terrorizing negotiations, and exceedingly unusual relationships.
I enjoyed it (although I'll read Ha'Penny and Half a Crown before I go on with his sequels) and think most people would enjoy it.