Robots or humans...plausible or not, I really enjoyed Old Man's War as well. It was a very accessible, enjoyable read. I don't read a lot of sci/fi (mostly fantasy, uf and mystery) but this was a good, fun read for me.
By the way, I don't mind stories with automatons carrying the bulk of the fighting. Where would Iain M. Banks's Culture novels be with out the Minds? (Although I have sometimes wondered whether the Minds keep the biological members of the Culture as high-maintenance pets.)
I was more concerned that the absence of fighting machines should not be seen as a fundamental error by an author, but a possible consequence of a rational, logical decision (in the case I was pushing, that you might not be able to trust them not to turn round and kill you).
If the use of biological soldiers was to be seen a grave error (of imagination? of good sense?), great books such as Dune could be consigned to the "it was good for its time" bin, which is ridiculous. To be fair, Herbert clearly stated why his universe was the way it was (but in a close-POV-dominated environment, characters are not obliged to think back over the history of their societies, particularly if they are at the sharp end of things.)
Not a comment on the book, but the responses: I think it's much better for an author to be love or hate (as Old Man's War seems to be) than liked in a lukewarm way. I think it was Stephen Lawhead's Grail series [hope I got the names right] which I remember here being generally liked but not in an enthusiastic way. Naturally I hope anything I write will be universally popular, but I'd go for it being rated 5 star and 1 star rather than 2 and 3, I think.
I don't love OMW or hate it; as I said in my review, it's a fun read which I finished in a couple of sessions. However, the plot is basically unoriginal and has logical flaws. As a result, I rate is as OK but unexceptional and unmemorable.