I just finished The Ghost Brigades by John Scalzi, sequel to Old Man's War.

I thought Old Man's War was a good book, although it didn't add much to the classic military SF (Starship Troopers, Forever War) it seemed to emulate.

Ghost Brigades is better in that regard. The first book focused on the future's version of the infantry while this book focuses on the special forces, an elite group of soldiers created just for this purpose. It can also tell a more involved story, since it doesn't have to do as much as the first book to describe the background universe. This convoluted plot involves a human traitor that is working with a few other alien races against the human army.

It also avoids a lot of "sequelitis" problems - there are very few overlapping characters from the first book, so it tells a new story and advances the background story as well, rather than just repeating large pieces of the initial book.