I like Joe Haldeman, I like his books, but this wasn't a good one. It only gets three stars rather than two because his writing style is so compelling.
In Old Twentieth Haldeman spends time setting up a potentially very interesting scenario with some interesting characters. Frustratingly he then expands on what in my opinion would have been one of the less interesting sub-plots and makes it the entire story at the cost of all the other potential avenues he could have explored.
The story is interspersed with vignettes which, while interesting in their own right, bear very little relation to and barely add to the main story. They serve merely to allow Haldeman to explore and write in specific historical eras, including of course his favourite topic of wars. In fact the Vietnam war vignette is where Haldeman most obviously comes alive, this of course being the war he fought in and was the inspiration for most of his writing.
The book is generally compelling and has good elements of humour. Being so short I read through it in an evening and enjoyed it. However, I can't help but feel he's missed a trick here. There are a dozen much more interesting and exciting plots he could have chosen to explore in the scenario he weaves. Consequently I was left feeling cheated and disappointed.