There's been debate over the years about what the worst science fiction novel ever published might be. Robert Coulson singled out an A.E. van Vogt novel, my father said it was a Geoffrey Hoyle book, and several people have pointed at Galaxy 666 by Pel Torro (which I recently found and will read one of these days soon). Dionysus: The Ultimate Experiment by William S. Ruben comes to mind. Them by Robert French is definitely somewhere in the top ten, though.